ENH: add a new init method for k-means

ENH: add a new init method for k-means

- implement the kmeans++ initialization (_kpp) as proposed by D. Arthur and S. Vassilvitski (2007)
- remove references to unsupported init methods ('uniform')
Also, please move it to the ``kmeans2`` docstring. This is a private function, therefore this reference won't show up in the html/pdf docs.