Converted mixPpcaDemoNetlab to python (#411)
Converted mixPpcaDemoNetlab to python (#411)

Closes #230,
I implemented the Probabilistic Principal Component Analysis mixture model following the research paper "Mixtures of Probabilistic Principal Component Analysers" by Michael E. Tipping and Christopher M. Bishop.
please visit pages 5-10 from http://www.miketipping.com/papers/met-mppca.pdf for the mathematics and the algorithm used in this code
I made the suggested changes:
1. Used the log sum trick everywhere
2. Changed the confidence interval to 2 standard deviations like in confidence_ellipse
probml/pyprobml,probml/pyprobml,probml/pyprobml,probml/pyprobml