New authentication API, support OAuth web flow

Besides supporting web flows, there are a couple other major changes here:

 * The `-requestAuthorization:…` methods have been merged with `OCTClient`'s constructors, since the multi-step login process was always a bit confusing. This will also result in more natural logic for invoking the web flow. 
 * You can now set the `User-Agent`, client ID, and client secret for your application via a class method on `OCTClient`. The global nature of it is a little gross, but this makes the constructors less verbose, and these are values that should never change during your app's lifetime anyways.

See the [README](https://github.com/octokit/octokit.objc/blob/oauth-web-flow/README.md) for examples. Resolves #31 and #75.

**To do:**

 - [x] Refactor `OCTClient` implementation to match the new interface.
 - [x] Update unit tests.
 - [x] Add tests for `NSURL.oct_queryArguments`.
 - [x] Add [authentication examples](https://github.com/octokit/octokit.objc/issues/75) to the README.

@mdiep @joshvera @jacksonh