This allows use to provide convenience methods for accessing the
paginations links that are returned in HTTP Link headers.  To expose
that, we'll need to return the Response from all API methods, which will
also allow users of the client to do any other kind of inspection of the
response.  This adds additional complexity to the API and will be a
breaking change, but seems to be the cleanest way to enable this sort of
thing.

Refs #22

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.

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

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

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 to prevent mismatch between tests and resulting
error messages.

also add specific error type checking for invalid JSON test

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".

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.