BUG: fix import issue with ``python -OO`` (stripping docstrings).

Docstring formatting at top-level leads to failed import when Python is run
with ``-OO``.
BUG: fix all issues found when running test suite with ``python -OO``.

Closes gh-2952.
MAINT: run Python 2.6 tests on Travis CI with -OO interpreter flag.

See gh-2952.
TST: fix test error with older numpy (comparisons with inf/nan broken).
TST: some cleanups of test files.  Remove "import nose" where possible.
MAINT: replace WarningManager with warnings.catch_warnings

Also replace some ``nose.assert_true`` statements with ``np.testing.assert_``.
TST: fix test for distance funcs that's failing for older numpy versions.