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.
FIX remove warnings from univariate FS

These warnings are issued practically always when using
frequency-valued or boolean data.

Switched to a stable sort to get reproducible results.
Merge pull request #2368 from emsrc/cosine_distance

Cosine distance metric for sparse matrices
Merge pull request #2369 from larsmans/no-warnings-in-fs

[MRG] remove warnings in univariate feature selection
make it possible to pass offsets for svmlight loading
WIP: add failing test to investigate dtype issue
more debug output for previous test failure
Rebased on top of current master
More tests for test_load_with_offsets with simpler tests first
Fix broken test for load_with_offset (zero_based heuristic failed)
switch from offset_min / offset_max to offset / length API
Make n_features mandatory for chunked loading