Add predict method to AffinityPropogation and MeanShift

This is a simple PR for adding predict() to AffinityPropagation and MeanShift.

For SpectralClustering, I'm not sure if it's possible to directly generalize to new data (it's always possible to fit a classifier with the labels obtained from fit_predict, though).

For DBSCAN, it seems like it should be possible but I'm not familiar enough with the algorithm so I didn't do it.