Downgrade version requirement of cython to 0.14

Cython default version on Ubuntu 12.04 is 0.14, in order to support this
version we needed to remove const attribute on signature in _code.pyx
and _code.pyd, because it was not supported.

This fixes issue #3
Merge pull request #6 from Chwing/master

Downgrade version requirement of cython to 0.14