Added cosine distance metric for sparse matrices

- Added 'cosine_distances' function to sklearn.metrics.pairwise.
- Added 'cosine' as metric in 'pairwise_distances' function.
- Corrected doc string of same function, because all metrics based on
the 'manhattan_distances' function (i.e. 'cityblock', 'l1', and
'manhattan') do currently NOT support sparse matrices.
- Added corresponding corresponding unit test.
added missing assert in unit test
Fixed doc string; compute cosine distance without copying matrix.
Merge pull request #2368 from emsrc/cosine_distance

Cosine distance metric for sparse matrices