EntityUserProvider should implement UserProviderInterface

I tried extending `EntityUserProvider` and ran into issues because it doesn't implement `UserProviderInterface`. This fixes that.
Expose entity manager to subclasses of EntityUserProvider

This is useful for creating and persisting a user that doesn't exist
Merge pull request #483 from lmcd/0.3

Expose $em and implement UserProviderInterface in EntityUserProvider
[Bugfix] Make error description detection compliant with OAuth2
The arameter 'error_description' should be also used to detect
error message, as in the OAuth2 documentation (http://tools.ietf.org/html/rfc6749#section-4.1.2.1).

Original code by: Olivier Maridat
Added italian translations
Adding createdAt property to serialization

The createdAt property was not serialized so the isExpired() method was not working as intended.
[Bugfix] Prevent notice issue when provider send error that callback was not confirmed
Merge pull request #486 from stloyd/bugix/notice

[Bugfix] Prevent notice issue when provider send error that callback was not confirmed
Add missing notes into CHANGELOG file
Merge branch 'hwi/0.3'