Add VonMisesFisher, a ContinuousMultivariateDistrib. for unit vectors

The [von-Mises Fisher distribution](http://en.wikipedia.org/wiki/Von_Mises%E2%80%93Fisher_distribution) can be useful for directional statistics. I've included a first attempt at `logpdf`, `rand` and `fit_mle`. I wasn't sure how to implement `rand!` to properly work with all the incantations of `rand!` in `src/fallbacks.jl`. Also, more correctness checks are needed.