Implemented setup.py

Making this a PR because it breaks the usual workflow, so everyone should know about this before it gets into master.

Now, you can do the usual `python setup.py install` or, preferably, `develop`, and it make `nengo_gui` importable, and will create a `nengo_gui` script that you can run from anywhere on the command line. Not sure how this works with getting an icon for Windows; if that's critical I can play around with it.

I kept the file named `main.py`; the way setuptools works is that it will create a launcher script automatically if you tell it that a particular function can handle it. So I just made that function `nengo_gui.main` (imported from `main.py`), but feel free to move it wherever makes sense.

Other changes
- Added version.py to track version number; v0.1.0-dev right now
- Make README an RST doc as that's what PyPI can parse
- Added myself to CONTRIBUTORS.md (@e2crawfo should do the same!)

Fixes #56.