Serialization and some improvements

This is  a work in progress... this PR pushes the Menu implementation closer to the Knp one.

- Support for fluid interface
- Serialization / Deserialization
- Tests
- Support for sf2.2 in composer

I tried also to extend ``Knp\Menu\MenuItem`` but this rapidly became more than I wanted to do -- but is this a good idea? The main problem I see is that it indexes the menu child items on their names. We could probably simulate the same thing.

Also I guess we can add travis for this now.