install_requires / requirements.txt

Update setup.py to set install_requires and update requirements.txt to
just install a development copy of the library.

Hey, I was really disappointed yesterday when I did `pip install wok` then `wok --help` and got an import error when trying to import `yaml`. I don't think `requires` is a kwarg `setup` even recognizes, so I changed it to `install_requires` and moved all the requirements from `requirements.txt` there and then made `requirements.txt` just install wok from the current working tree.

Let me know what you think.