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
Change to:

As project size increases, consistency of the code base and documentation becomes more important. We therefore provide guidelines for code and documentation that is contributed to scikit-bio. Our goal is to create a consistent code base where it is easy to find relevant functionality (and to determine when functionality that you're looking for doesn't exist), you can trust that the code that you're working with is sufficiently tested, and names and interfaces are intuitive. 

**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, or have a suggestion for something new to include, you should [create an issue](https://github.com/biocore/scikit-bio/issues) to initiate a discussion.**