setup.py: Include a dependency link for PyCAPTCHA 0.4.

PyPI only has versions for Python 2.3 and 2.4. Modern installs that
don't use our PPA to get a debianized copy of this library fail when
trying to resolve the setup.py install_requires dependencies. This fixes
the problem by using a reddit-hosted tarball of the 0.4 source (taken
directly from the source SVN repo).
setup.py: Remove obsolete line.

This should've gone away in 8b7b3a307b03d8ac8323c84d1adcf759fb9fb4d5.
setup.py: Ensure we get WebTest <= 1.4.3.

The 2.x series of WebTest requires WebOb >= 1.2 which won't work for us.
Use `cythonize` to manage translation of .pyx -> .c.

This removes the need to run `make pyx` before running setup.py on an
initial install which had stopped working due to various imports needed
for the makefile. Additionally, cythonize is capable of tracking
dependencies the cython files have, such as external headers or other
cython files.

I had to upgrade setuptools to distribute to get v0.6.16 which is the
minimum required to properly use `cythonize` due to a bug in setuptools:

https://bitbucket.org/tarek/distribute/issue/195/