Added repositories.go and repositories_test.go.
Updated the README.

Elaborated on the use of Trello.
Implemented GitHub.Repositories() and User.Repositories() methods.

There is some strange issue happening where only one of the returned
JSON objects is getting unmarshaled. :/
Started adding some more fields to the Repository struct; these fields
were gleaned from manually calling GitHub's API.
Set the default page size (30) as a const.

This is in a pre-planning effort to hopefully make pagination easier.
Reverted to making the repositories initially have a size of zero.
Changed some of the logging output.
Explicitly set the Accept header in the request.
Finished mapping out the fields from the JSON response for a repository,
to the Repository struct.

Oddly enough, this totally fixed the "only one repository is showing up"
issue!
Added empty files for implementing the organizations endpoints.
Implemented the (*GitHub).Organizations() and (*User).Organizations()
methods.
Implemented the (*GitHub).GetOrganization() method.
Added a private field to the Organization struct, for holding the
currently-active *GitHub object.
Merge branch 'organization-endpoints'