Fixes #486.
Closes #488.

Change-Id: I31ff003552dbffaca78c740b89d54150b81c525c

Fixes #420
Closes #423

Fixes #397.

Change-Id: I30ce336e3a36cf5758b9bf52b9dd77f7c1df5f27

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

see 92bbb1ce51

This adds support and integration tests for editing branch protection.

Add a new error type, TwoFactorAuthError, which identifies the need to
include a one-time password when using Basic Auth for a user with
two-factor auth enabled.

An example of using both the new transport and error type can be seen in
examples/basicauth/main.go.

fixes #258

RepositoryListByOrgOptions doesn't have a `Sort` field

Fixes references to depreciated goauth2 libaray and replaces example
with the recomended oauth2 implimentation.

- minor updates to README
- expand package docs to include more information on authentication, as
  well as new sections for rate limiting and setting pointer values.

Now that we're using anonymous struct fields for Rate, go 1.1 really is
required.