Added caching layer

To improve performance, the `ApiDoc` metadata is being cached and automatically invalidated when any of the routing files and related controllers are updated.

Added a few tests for this. Please let me know if they are insufficient and how I can improve them.
I'm aware that it could be done like that and use `$container->getParameterBag()->resolveValue(...)` on it, however I assumed that the cleaner approach is as illustrated on this cookbook entry: http://symfony.com/doc/current/cookbook/configuration/using_parameters_in_dic.html