Added support for manipulating webhooks for repositories

Refs #22

Refs #22

add quotes around json tag values in order to be compatible with
reflect.StructTag.Get

Repository service:
    ListLanguages   : Allows a user to list the languages for a repository
Activity service:
    ListStarred     : Allows a user to list the starred repositories for a user

This provides the structs needed to unmarshal the "old-style" post
receive hooks that are sent for push events.  Because the formats
differ in sometimes subtle ways from the rest of the API, some types are
duplicated.  I suspect we may also need to duplicate the Repository
type at some point, since there are some differences there as well.  For
now this is just using the normal Repository type.

Thanks to both @wlynch92 and @imjasonh who kept pushing on this and
provided the original patches that this is based on.

Added a timestamp struct to handle different incoming time formats from
GitHub. Only modified the repo struct to reflect this new time struct
for now since it is currently the only location I am currently aware of
where this is an issue.

Issue: #1

The main purpose of adding this (aside from simply implementing the full
GitHub API) is to establish the basic Commit type, so that we can build
around it for things like Events (see #4) and WebHooks (see #20).

There shouldn't be a SHA in the URL
Also rename 'user' to 'owner'

- complete the rename from TreesService to GitService.  For example,
  renaming Get to GetTree, and Create to CreateTree.

- rename GitTree to TreeEntry.  I really have no idea why GitHub chose
  to use the field name "tree" for this, since these aren't necessarily
  trees in the git sense.  Looking at it more closely, these are really
  just the entries within a tree, hence the rename.

- simplify several tests and shuffle some things for consistency with
  the rest of the library

Because the reset date is expressed as a Unix timestamp (rather than an
ISO 8601 timstamp), we can't directly unmarshal the JSON response.
Instead we have to do a little weird indirection to unmarshal it as an
int first, and then convert it into a proper Time object.

fixes #18

Since the library has received external contributions, Google is no
longer the sole copyright holder.  Update the boilerplate at the top of
each file to reflect that, and add AUTHORS and CONTRIBUTORS files to
track this information.

there's no real need to use pointers here

update Do() to update client.Rate on every API call. Fixes #12

This includes tests for relevant methods. Fixes #14.

This includes most API methods on pull requests (with the exception of
listing commits and the merge functions), as well as all of the methods
for pull request comments. (Fixes #13)

This also fixes a few oversights in the issues API.

This adds basic methods for working with issues, assignees, and issue
comments.  Issue events, labels, and milestones are not included in this
commit. (Fixes #7)

add methods for testing request method, request form values, and URL
parse error

golint doesn't like the underscore in "url_"

add test to check for required fields, as well as test for default vs
custom transport.

This allows for making unauthenticated requests subject to your
application's rate limit (as opposed to the lower rate limit for
anonymous requests)

use temporary variable for expected value to prevent mismatch between
tests and resulting error messages.

Also properly return a 204 response on methods that don't include a
response body.  This has no actual effect on current tests, but more
accurately reflects what the GitHub API really returns and might make a
difference in the future.