Implemented User.GetPublicKeys().
Added methods GitHub.PublicKeys() and GitHub.GetPublicKey().
Added the GitHub.httpPath() function for making HTTP PATCH API calls,
and added special handling for an HTTP 422 response status code.
Ran gothub.go through `go fmt`.
Implemented the GitHub.AddPublicKey(), UpdatePublicKey() and
RemovePublicKey() methods.

NOTE: The test for UpdatePublicKey() fails. The error codes for the
PATCH /user/keys/:id endpoint are undocumented. The "code" value
returned by the API is "custom", which is not documented anywhere.

I will probably have to submit an issue to GitHub for this.