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')
This won't work if ``data`` is a 1-D array (which is allowed). Besides fixing this, it would be good to add a testcase with 1-D input (shape ``(M,)`` not ``(M, 1)`` as you have now.