implemented fast listKeys for gridfs

as proposed in #118 . Filesystem now checks if adapter has listKeys method (no need to implement it for all adapters, imho, since filesystem/ftp traversing is not subject to optimization), gridfs implements listKeys via MongoRegex.

I also added a typehint to FunctionTest $filesystem property.

@l3l0 , please check it this PR complies to your thoughts about Gaufrette. If it's ok, I'll propapbly implement it also for doctrine via LIKE (is also should be applicable to S3 / Rackspace, but I've no posibillity to test that).