Addition of Vega syntax generation

The major update is the addition of Vega syntax via incorporation of the Vincent project: 
https://github.com/wrobstory/vincent

None of the original API/syntax for building/showing figures has changed- you can still build figures from the ground up using d3py.geoms. Now you can also build them with vega syntax. 

I also did some code commenting and PEP8 cleaning, started to build some more comprehensive tests (need a lot more work), and moved some of the methods in figure.py around so that the class logic flows better for the first-time reader.