Add coding guidelines

These are the coding guidelines as updated from what originally was in
PyCogent. I took a pass through the document and updated where I saw fit,
however I know that people will like to add things that are currently not being
covered, so let me know and I'll add/change as needed.

The section that needs the most attention is the testing section. I updated it
but I know there might still be things that are missing as I'm not super
familiar with nose.

I've also added a minimal section about git that mainly points to other well
known and well written git guides.

Fixes #272
I suggest changing this sentence to:

*The structure of your module should be similar to the example below. scikit-bio uses the `NumPy doc`_ standard for documentation. Our `doc/README.md`_ explains how to write your docstrings using the `NumPy doc`_ standards for scikit-bio.*