WIP Grid API overhaul

:warning: do not merge until "WIP" disappears :warning:

For the purposes of:

* Simplifying grid api;
* Closer similarity to existing object metaphors;
* Greater flexibility;
* Incorporating feedback from other developers and designers;

I'm making a fairly early, but major, change to how grids are constructed and how pixel locations are returned. 

Instead of `xTilt` and `zTilt` on the grid, which serves as a way to create a kind of 3d perspective, we're going to build an actual camera object with X and Y rotation.

![camera](https://f.cloud.github.com/assets/175515/1103362/fc9116ba-18a3-11e3-9f71-76ddf0df1cdc.png)