Added ext-mbstring to composer require.
Merge pull request #161 from SammyK/mbstring-in-composer

Added ext-mbstring to composer require.
Adding developer site doc content, needs lots of updates.
First step to refactor response/request
Add FacebookApp entity
Add FacebookRequest entity
Add FacebookResponse entity
Add FacebookBatchRequest entity
Add FacebookBatchResponse entity
Add FacebookClient class
Update FacebookResponseException to use FacebookResponse
Add Code entity
Update AccessToken entity and associate it with a FacebookApp entity
Add DebugAccessToken entity
Update SignedRequest entity and associate it with a FacebookApp entity
Update helpers to use FacebookClient and FacebookApp
Add request and batch request builders
Clean up