Add Probabilistic PCA example (code already exists)

[twiecki]

I coded this up a while ago, pretty simple model but I think it's an interesting (and commonly referred to) model: https://gist.github.com/twiecki/713cf58f5fa37332d0cda48a7c798016

Probably just needs some more text and explanation.

[drbenvincent]

Would be good to show with 2 factors, and also one with non-orthogonal factors to show limits of PCA and motivation for ICA perhaps.