Fix URL for CreateTree

There shouldn't be a SHA in the URL
Also rename 'user' to 'owner'
Add methods for git commits

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).
EventsService with one API method, ListPerformedByUser, and Event struct
Add support for typed and untyped event payloads
Add timestamp struct to handle different formats

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
initial support for post receive hooks

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.
Add Billy Lynch and Jason Hall to CONTRIBUTORS
add Repositories.Edit method
minor updates to event tests
Added ListStarred and ListLanguages functions

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
add Jameel Haffejee to CONTRIBUTORS and AUTHORS
fix go-vet warnings

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

Refs #22
pass page parameters on issues.list method

Refs #22
Implemented hook functions

Added support for manipulating webhooks for repositories
Add Stian Eikeland to AUTHORS and CONTRIBUTORS