setup.py: add requests to setup_requires

See #16 and
http://stackoverflow.com/questions/27497470/setuptools-finds-wrong-package-during-install

Multiple hours of 100's of Google searches eventually took me to this
numpy thread:
    https://github.com/numpy/numpy/issues/2434#issuecomment-65252402
for which they had to add numpy to `setup_requires` AND
`install_requires` to fix a similar issue. Doing this with requests
seems to have worked. However, I am somewhat skeptical as posters on SO
indicated they did not see our problem in the first place. I may just be
getting lucky.

I tried on my Mac as well as on Ubuntu (dbserver02), and am able to
install from a clean virtualenv on both systems.

Fix #16 for now.