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')
Can you complete this reference? I think it should be:

    D. Arthur and S. Vassilvitskii, "k-means++: the advantages of careful seeding",
    Proceedings of the Eighteenth Annual ACM-SIAM Symposium on Discrete Algorithms, 2007.