Ability to log with email only

Hello @stof,

I thought it could be useful to log with email only as we can already log with username and username/email.

The class ```FOSUserBundle / Security / EmailUserProvider.php``` has been renamed to``` FOSUserBundle / Security / UsernameOrEmailUserProvider.php```

```FOSUserBundle / Security / EmailUserProvider.php``` is now the class that allow logging via Email only.

Tests seem ok (correct me if I am wrong, first time I am doing this :-) ).