Went back to using github.com/mrjones/oauth.

Matt Jones (mrjones) was kind enough to merge my changes in.
Added a *Client.CopyTo() function.
Added doc comments for the *Client.Do() function, and changed the call signature to allow for custom URL-encoded parameters.

The new `params` argument is just a string-string map.
Fixed up the tests.
Added several more structs, and moved some of the existing ones around.

I have started grouping the structs, in files, in a similar fashion to how the API documentation groups the endpoints.
Added structs for the contacts endpoints.
files: added structs
sync: added structs for accounts/:id/sync endpoints
threads: Added ThreadList and Thread structs.