Implement a Horseshoe Gibbs sampler step method

[xjing76]

add in Horseshoe Gibbs sampler along with unit tests so we can start moving forward with a PyMC step method implementation

[brandonwillard]

This addition should follow the basic design/approach/template of pymc3_hmm.step_methods.FFBSStep (e.g. implement all the same methods, perform similar (symbolic) input validation and setup, have an informative docstring with some math details, etc.)—and also be added to pymc3_hmm.step_methods. The same goes for its tests.