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 recommend adding this line:

**As scikit-bio is in pre-alpha release stage, our coding guidelines are presented here as a working draft. These guidelines are requirements for all code submitted to scikit-bio, but at this stage the guidelines themselves are malleable. If you disagree with something presented here you should create an issue on the [issue tracker](https://github.com/biocore/scikit-bio/issues) to initiate a discussion.**