Add additional mcmc samplers

[jejjohnson]

I think it would be good to have additional MCMC samplers. I know that pymc3 has the metropolis-hasting samplers and I've seem some code online that might be helpful. I would like to add this as an option in our framework.

Some examples:

• metropolis-hastings - native to pymc3, we can just use the code there (another example)
• emcee - affine-invariant mcmc sampler; based on Dan's example, it seems like it should be straightforward
• dynesty - Nested sampler; I imagine it's the same as the above as the API is the same