update LICENSE to include GitHub CC-BY license

fixes #11
first pass at pull request methods

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.
fix typo in IssueComment and return more API error data
Add ListEmails, AddEmails, and DeleteEmails for Users service.

This includes tests for relevant methods. Fixes #14.
recommend that contributors run golint
remove note about squashing commits

Squashing is a little advanced, and easy to screw up with github pull
reqeusts.  If needed, I generally squash them myself anyway.
add Rate to github.Client

update Do() to update client.Rate on every API call. Fixes #12
pass and return user emails as slices

there's no real need to use pointers here
Update boilerplate to reflect shared copyright

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 copyright notice in LICENSE file
move AUTHORS and CONTRIBUTORS to root directory
Adding skeleton of gist support
Fixed issue with type of ID, and moved `owner` back to `user` in the json.