Added Social blocks

Added Facebook Social Plugins

Fixed typo

Added Twitter buttons

Fixed PR

Fixed PR

Moved BlockBundle dependency as a suggestion

Added checks for SonataBlockBundle presence

Removed include_sdk option

Updated doc

Fixed PR

Fixed tests requirements

Fixed tests requirements

Refactored code

Updated doc

Fixed PageBundle dependency in tests

Fixed dependencies
Merge pull request #29 from sylvaindeloux/SONATA-214

Added social blocks