Add a setup.py file.

I'm going by cargo-cult here, mostly just copying stuff from other
python that are lying around (especially pygments, polib, and babel).
But I also looked at:

http://www.diveinto.org/python3/packaging.html
http://docs.python.org/2/distutils/
https://pythonhosted.org/setuptools/setuptools.html

As part of this process, I realized I couldn't name the executable
script the same as the python directory, so it's now i18nize-templates
(- instead of _).  Ah well.

Test Plan:
I ran
   python setup.py test
and
   python setup.py build
and they *seemed* to do reasonable things.  We'll have to see when I
try to upload this to pypi.