Replaced Deprecated Function Call

`getEntityManager` is deprecated since Symfony 2.1. `getManager` should be used instead.
Merge pull request #140 from sprankhub/patch-1

Replaced Deprecated Function Call
added PreviousStepInvalidEvent and PreviousStepInvalidEventListener
Merge pull request #143 from craue/previous-step-invalid-event-listener

add PreviousStepInvalidEvent and PreviousStepInvalidEventListener
removed debug code from tests
updated Travis config
Add yaml example in documentation

Double quote for yaml
Merge pull request #144 from WedgeSama/master

Add yaml example in documentation
cleaned up code of some tests
moved library-related code into the dedicated FormFlow folder
use a compiler pass to register translation files in their new location