Allow use of base class SimpleUser\User as well as child classes of that class

Add tests to ensure that UserManages supports and can refresh both.

See: https://github.com/jasongrimes/silex-simpleuser/issues/8
Merge pull request #9 from parsingphase/fix-jasongrimes-silex-simpleuser-8

Allow use of base class SimpleUser\User as well as child classes