ENH: Add cythonized subsample
ENH: Allow it to overwrite the pure Python implementation.
TST: Allow testing of both modules
ENH: Add C source code.
ENH: Installation
MAINT: Add future to requirements.

(https://github.com/PythonCharmers/python-future)
TST: Fix travis
BUG: nose should use the installed skbio

nose was importing skbio from the source directory, not from where
pip was installing it, so now that there's a compiled module it doesn't work.

Fixed by installing in-place (aka develop/edit mode).
STY: remove extra line (thanks @wasade)
ENH: 15% more speed (thanks @wasade).
MAINT: tidy up declarations, including right types for indices.
And of course update .c code.
TST: Add another build that regenerates '.c' files from '.pyx'.
TST: One more space...
TST: silly overlook
ENH: refactor cython extension, no need to duplicate everything.
ENH: fast path applies to the Cython extension too.
Merge pull request #262 from Jorge-C/cython-subsample

Cython subsample