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
extracted methods for adding/removing route parameters into a separate class
added support for the "redirect after submit" pattern