Add Python requirements file.

I added the Python requirements.txt file with the Sphinx-doc module necessary to build documentation.
Use the file with ``pip`` command:

    pip install -r requirements.txt [--user]

Fixes ``make`` build error:

make[2]: Entering directory `/home/vmlaker/work/git-h2o/h2o-docs'
sphinx-build -b html -d build/doctrees   source build/html
Making output directory...
Running Sphinx v1.1.3

Extension error:
Could not import extension sphinxcontrib.fulltoc (exception: No module named fulltoc)
make[2]: *** [html] Error 1
make[2]: Leaving directory `/home/vmlaker/work/git-h2o/h2o-docs'
make[1]: *** [dw_2] Error 2
make[1]: Leaving directory `/home/vmlaker/work/git-h2o'
make: *** [nightly_build_stuff] Error 2