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, maybe instead of ``init[i]`` you can use ``init[i, :]`` everywhere (same but easier to read).