Social media authenticator #117

Here is a proposal of a facebook account athenticator.
To test this patch, you need to :
* Create a facebook application, in the settings make sure to have "com.donnfelker.android.bootstrap.authenticator" as the package name, and FacebookAuthenticatorActivity as the activity name
* Grab the facebook app ID and update app_id in strings.xml with this ID

If there is any issue, please let me know

Regards
Proximator