Guarantee oriented simplices for 2D delaunay

This commit updates the existing Delaunay code in order to ensure
that 2D triangulations maintain a consistent ordering. This
is similar to specifying the 'i' flag in QHull. This brings
the behaviour inline with Matlab, and is the most sensible output
for 2D triangulations.

This is achieved by swapping the first and second indices if
the orientation of the facet is clockwise. The logic is written
to try and minimise the impact on higher order triangulations.
Remove the 0 when setting qh_ORIENTclock

The default value for an enum is 0 so the setter is redundant
Fix tests to ensure counterclockwise orientation
MAINT: raise error on import for unsupported Python versions.
TST: special: skip some tests on Py3 + Numpy < 1.6 due to Numpy bugs
Merge pull request #2955 from patricksnape/branch_delaunay

ENH: spatial: guarantee oriented simplices for 2D delaunay
DOC: spatial: mention orientation of simplex points in Delaunay
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.
Merge pull request #2964 from rgommers/fixes

BUG: collection of minor fixes