GitHub API docs: https://developer.github.com/v3/oauth_authorizations

Fixes #348.
Closes #388.

Change-Id: I795b1c0e10bde10a1bc9aaa4594af4c91e07962d

Note that this is an API-breaking change but should have minimal
impact on users of this package due to the nice inference
properties of the Go programming language.

Bumped `libraryVersion` to `2` due to API-breaking change as
discussed in #376.

Fixes #180.

Change-Id: Ib386135e6b8f306d1f54278968c576f3ceccc4e7

Fixes #334.

Change-Id: I9b10ae5e7679f5196778d9eb3ded6168d7facfc3

- add new Scope type with values for each of GitHub's current scopes
- add String method to Authorization types
- doc updates

Fixes #119
Fixes #241