Use marginalized mixture classes for DP example

[AustinRochford]

I have updated the Dirichlet process mixture model example to use the marginal mixture model classes.

@twiecki I am not sure how to rebuild the public docs after merging this.

[springcoil]

I think @twiecki has to rebuild the docs himself.

[twiecki]

Very cool, now live at: http://pymc-devs.github.io/pymc3/notebooks/dp_mix.html

(I just do make html and then do ghp-import to copy to the gh-pages branch).

@AustinRochford Is convergence faster than before? Seems like the old-faithful alpha trace still takes a bit to settle. Have you tried more ADVI samples and advi_map initialization?

[AustinRochford]

@twiecki yes, it is faster than before; more ADVI iterations did not seem to improve things; I did not try advi_map.