Implement Maxwell-Boltzmann distribution helper

[ricardoV94]

See https://github.com/pymc-devs/pymc/blob/ec4407d3db2c8ebe77e62c65f2eda3f5703e6279/tests/distributions/test_continuous.py#L994-L1021

We already implemented the Chi in #239

[williambdean]

Does @twiecki still want to implement?

[ricardoV94]

Does @twiecki still want to implement?

I don't remember any disagreement about implementing it, just how it should be done. Feel free to take a stab at it

[ricardoV94]

Closed via #261