Compatibility with Elastica 0.20.x (incl. Serializer support)

fixes #257 
i would expect that there are still some issues here and there .. but the test suite passes and the little stuff we do in our app also still works. note this also adds in the Serializer support that @leahaense created here https://github.com/liip-forks/FOQElasticaBundle/compare/master...serializer_support migrated to the Context support that is now in the serializer.