Added a support for OAuth 2.0 Grant Type.
For instance, Grant Type is used in Facebook App Login

Change-Id: Id0abb4ddedd205428e63b43c5d7a9bfe5ad87be4
Added a sample for Facebook App Login and Graph API (insights) call

Change-Id: Ie719bb118a98d4794d5da6c9aad2a281c7caf413
Refactoring. Support for OAuth 2.0 grant type based on 
draft-ietf-oauth-v2-16

Change-Id: I833f337776fc82fc3dd8af0d2ec68e74ec0ef489
Merge remote-tracking branch 'origin/master'

Conflicts:
	src/main/java/org/scribe/builder/ServiceBuilder.java
	src/main/java/org/scribe/builder/api/FacebookApi.java
	src/main/java/org/scribe/model/OAuthConfig.java
	src/test/java/org/scribe/model/OAuthConfigTest.java