Add msgpack as submodule for pandas
Let's get the ball rolling on better high perf serialization. Note this is based on the 0.3.0 release of msgpack-python, should be updated whenever another stable release of the library is made. 

@wesm is there a reason you are using `pytest`? rather than nose
 Unit tests are directly from the msgpack module, should be adapted to nose I guess
 closing as this is rolled into #3831
