Improved setup.py and potentially fixed issue 6

Hello,

Zenlib still had some issues building on Ubuntu so I worked on the setup.py script (see below) and added installation instructions in the repository's readme.md so that interested parties can install the library quickly.

The original issue: zenlib depends on networkx but networkx wasn't listed in the install instructions nor was ``setup.py`` fetching any dependencies. I hypothesize that the commenting out of the ``install_requires`` was originally due to setuptools and cython's incompatibility which led to the use of ``distutils.core`` as a stop-gap. To fix this I've clearly indicated the dependency on [distribute](http://pythonhosted.org/distribute/http://pythonhosted.org/distribute/) which fixes this [issue](https://bitbucket.org/tarek/distribute/issue/195). Dependencies can now be listed again! It seems like distribute is [getting traction] (http://guide.python-distribute.org/installation.html) so I don't think it is such a far-fetched solution.

I also believe this fixes #6 as the problem there was due to missing .c files which would not have been built by the ``setup.py`` script using regular setuptools.

Hopefully this makes sense and helps make zenlib easier to install.