Distributions should distribution's (possessive)
Merge gaussian_mixture_sampler.py into IPython notebook

This is because it doesn't really make sense to have the gaussian
sampler subclass from the rejection sampler -- it is better to just
instantiate it directly.
Fix math error in equation for gaussian distribution
Be more explicit about what PDF means in docstrings
Make conditional in draw() clearer
Be clearer about what np.empty does
Better variable names in plot method
Add discussion about log-space and underflow
Rename X --> x
Add a few TODOs
Use more descriptive variable names, and include math in comments
Add TODO about variable name discussion