Added CONTRIBUTORS.md
Added abstract interface
Transformed email and Google authentication into two factor providers
Register two factor providers
Fixed PHPDoc annotation
Fixed service name
Moved session logic
Configuration option for entity manager
Updated documentation
Added provider alias
Moved class to new namespace
Introduced AuthenticationContext object
Removed old method
Renamed method
Simplified methods
Added documentation on custom authentication providers
Updated documentation
Merge branch 'abstract-interface'
Updated UPGRADE.md