Register Isotonic package for isotonic regression

I got some nice perf wins from improving `scikits-learn`'s implementation of isotonic regression  (https://github.com/scikit-learn/scikit-learn/pull/2944), so thought I'd convert it to Julia as a learning exercise. 

The code is (fairly weakly) unit tested, and there are some benchmarks on an [iJulia notebook](http://nbviewer.ipython.org/url/gist.githubusercontent.com/ajtulloch/9456289/raw/7a3429ffdb155d5f652f241475af9fdf4a28d2d1/Julia.ipynb).