[MRG] Kmeans memory problem

ENH: Precompute distances only if overhead is below 100MB

Precomputing distances can consume a lot of memory if several jobs are
used. With 8 jobs a dataset of shape (1e6, 2) and n_clusters=100 the
precomputed distances will consume 3GB of memory while the dataset only
consume 15MB (Assuming double precision is used).

since 'auto' is the default the other tests should already cover that code path. There are also test without the pre computing the distances. So I think everything is covered