BackgroundShader implementation and viewportStart accessor method

Hi jjoe64-

First thanks for graphview!

I added a background shader for my weather app because it shows 7 days of weather information on an hourly basis, and so highlighting every other 24 hours really helped show day boundaries at a glance:
![screenshot](https://cloud.githubusercontent.com/assets/7925634/3318085/9aa05872-f713-11e3-9373-0688e6faa5f2.png) It has a simple interface, so maybe it would be useful to others as well?

I also added the viewportStart accessor method so that I could ensure the viewport range stayed the same across lifecycle callbacks (eg.: screen rotations).