Enable sampling from variational distribution

[dereklhansen]

No description provided.

[dereklhansen]

We demo a simple procedure where we draw 5 samples from the variational posterior, calculate the likelihood for the whole image, and pick the one which maximizes likelihood (still need to add in prior information).

On the first example 300x300, precision increases from 0.8 to 0.9 without dropping recall.

On the second example 300x300, both precision and recall drop

On the entire image, precision increases a little while recall drops quite a bit (0.97 -> 0.89)

[jeff-regier]

Will this PR close #363?

[dereklhansen]

Closes #363.