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:

*If the variable is the name of the input file, call it ``infile_name``*

to:

*If the variable is the path of the input file, call it ``input_fp``*

That's a really common example in practice, so might as well go with it.