Added ability to query message store by ObjectId (python only for now).

Also added some little bits of logging.
Now returning from query srv
Added ObjectID into meta on query return
Made id query return single element.
Returning id correctly from service call.
Changed to get the deserialisation length from the vector length.

This removes the bug where variable length types were incorrectly deserialised. Thanks to @nilsbore for reporting the bug.
Adding an initial rostest for the message store proxy.
Adds a service to delete message by ID
Adding delete function to MessageStoreProxy and using it in unittest.
Added queryID to C++ side. Insert operations now return IDs. This closes #29.

Minor formatting.
Added constant for empty bson obj.
Added delete to example script.
Query methods now only return true when something was found.
Added test class for cpp interface.
Renamed to match convention.
Added rostest launch file.
Merge remote-tracking branch 'upstream/hydro-devel' into hydro-devel

Conflicts:
	ros_datacentre_cpp_client/include/ros_datacentre/message_store.h
Merge pull request #36 from hawesie/hydro-devel

Added c++ delete function and tests