Bearer Token Support

[jleandroperez]

Details:

• Updates WPComOAuthController so that the user is authenticated via Bearer Token.
• Fixed a callbackBlock setter (that definitely needed a copy call)

Testing:

1. Install WPiOS and Sign Into a dotcom account.
2. Add a new testing WordPress App Here
3. Open this url in Safari.app: wordpress-oauth-v2://authorize?client_id=[App ID]&redirect_url=[Callback URL]

As a result, WPiOS should be onscreen, and the user should be asked whether if he wants to authorize or not the app.

Note:
WordPressAppDelegate's setupSingleSignOn method should be patched, so that the authToken is initialized.

Closes #34. This is a blocker for this WPiOS issue.

/cc @sendhil

(Thanks in advance!).

[sendhil]

Very nice!