Read in requirements from requirements.txt

This pull request solves two problems:

1) `gnureadline` shouldn't need to be installed on linux systems, since the version of readline that they have is GNU by default.
2) Reduces code duplication by reading in the requirements from `requirements.txt` rather than having them separately in `requirements.txt` and `setup.py`.

The only downside is that then `gnureadline` isn't present at all in `requirements.txt`, but I think that should probably be ok, since that should only be a problem when you explicitly do `pip install -r requirements.txt`. If you install psiTurk from `pip` normally (i.e. `pip install psiturk`), or use `python setup.py install`, it will use the requirements in setup.py. An alternate strategy would be to keep `gnureadline` in `requirements.txt`, but remove it in `setup.py` if the system is linux. I'm happy to change it to do that if that's preferable.

Sigh, python packaging.