Add EdgeRenderer functionality

Excellent library, exceedingly well-done and useful.

I have a case where my "above" view contains an element that doesn't quite span the full height of the screen. I can't show much, but here's the left-top side of the screen:

![edge](http://f.cl.ly/items/0P431J1K3i2J1L2J3H1M/Screen%20Shot%202012-10-06%20at%205.47.08%20PM.png)

The white is the top part of the view, fitting some variable content, the brown is the bottom. That cardboardy-brown is also the background of the `SlidingMenu` below view. Therefore, it was desired to have the shadowed edge along the white part and a bottom drawn down at the bottom of that white part (top ended up being a 9-patch and bottom is a simple png). 

I extended out to allow the calling of an `EdgeRenderer` for the `CustomViewAbove` in place of -- or in addition to -- the standard shadow drawable canvas draw. Figured I'd send it your way to see if you found it useful to include in the standard repo.

Thanks again for the work.