Refactoring extension to use an OAuth2 RS

More info at: http://docs.webplatform.org/wiki/WPD:Projects/SSO/MediaWikiExtension

* Complete rewrite, using OAuth2 protected Resource Server
* Set in place use of Composer
* Require external component: Guzzle that’ll handle async requests
* Queries an OAuth2 Resource Server with minimal profile data
* Reads from profile data e.g. {username: 'Jdoe', fullName: 'John Doe' } and creates/use local account
* Overloads Special:UserLogin, Special:UserLogout
* Handling change password, removing from preferences redundant info
* Use memcache to keep state
* Using MW i18n methods, moving messages to it, making up english, then translating to french
* Moved validation responsibility to FirefoxAccountsManager

Change-Id: If5d3caec9b05b8362190f8ef30658efd135cdb4c