Implement BayesianMarkovStateModel

Clear PR with squashed history for `BayesianMarkovStateModel`.

- [x] Implement sampler
- [x] set `BayesianMarkovStateModel` API
- [x] Add docs
- [x] Add example
Honestly, I think relative imports would be better, but when you mix the two, it gets messy with `nosetests` running inside the source directory. Because `nosetests` will import the version in the source directory, as opposed to the version in the install directory. This usually isn't a problem (they're the same files, etc) except with cythoned fiiles, because the .so files aren't present in the source directory.