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

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.

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

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)

golint doesn't like the underscore in "url_"

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

if we have an error reading the body of an API error response, still
return an ErrorResposne with the original response struct.  Simply
returning a string stating that we couldn't read the response body
doesn't give the caller anything actionable to work with or attempt to
process.

This now aligns with the (pretty strong) golang convention of the
package name matching the path basename

this is particularly important for methods like
Organizations.CheckMembership which return a 404 to indicate a
successful response of "false".

also minor doc cleanup

This commit adds the core library funcationality and establishes the
general calling style and testing structure.  Only a few GitHub API
methods related to organizations and repositories are included in this
first commit, mainly to cement the calling style.