Allowing callables for mount
Allowing callables for mount

- Added the ability to pass callables into the controller collection
    mount method.
- Added a controllersFactory parameter into the controller collection
    which is a callable that is used to create a new controller for the
    mount method.
- Updated documentation to show usage of new callables in mount and
    recursive mounting.
- Added appropriate tests

Signed-off-by: RJ Garcia <0eb59c29b4a2ba03805b274ee15e75434b5e9157@bighead.net>

silexphp/Silex-Providers