Initial commit for sampler exampler

This includes a basic implementation of a generic rejection sampler, and
a more specific example of how a rejection sampler might be extended for
a specific use case.
Fix typos in docstring for RejectionSampler
Take the log of the y value, rather than exp(p(x))
Rename p to logy, because it is less confusing
Add some error checking to RejectionSampler.plot method
Flesh out README
Add link to nbviewer for the IPython notebook
Change uniform proposal to be between -4 and 4
Add a "seed" parameter to sample, to enable reproducibility
More updates to README

Better formatting for "why the log-PDF"
Add equation for Gaussian distribution and mixture of Gaussians
Add some possible extensions
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