Cython is optional.  When deployed, Cython is never used by default.

Cython is used by default to build *.c files for developmental versions.
TravisCI tests were added to verify building works without Cython.
Define version in "cytoolz/_version.py".  Add development requirements.
Python 3 compatibile `execfile`-like functionality
Merge pull request #14 from eriknw/cython_optional

Make Cython optional.  Use *.c files when deployed.