Cython as a build dependency

Use Cython at build time, rather than having all those *.c files committed in. The c files are rebuilt only when necessary (by comparing file content hashes). The hash DB and the generated files are shipped in sdist, so that building from tarballs works out also without having Cython installed.

All auto-generated .pyx files are committed in, however, as at least some of the autogen scripts require stuff like Mako be installed (this situation can probably be changed with a little bit of work, so that we don't need to have any autogen stuff committed in).

Not sure how this will play along with pip et al.