Grid system

This adds a 'grid' component which keeps track of objects on a specific scenario's grid (including inventoriables), as well as the boundary values for each space, albeit in separate arrays. So, any moving objects don't have to run through a list of inventoriables to find a collision, they only have to check the one space from the grid. 

Each scenario would have its own grid to define boundary values and where certain objects are located (the objects are currently added to the scenario's grid in `scene.js`).

Does this look like it would work well with the scenario system? @eric79 @BrettASwift 
We talked about this on the phone, but I would move these properties out of index and into the .yaml.

@BrettASwift, do you think that there's a way we could avoid specifying the scenario in the scene, and instead just have it be something that gets loaded at runtime?  Thinking ahead to when we have a lot of scenarios, and when users are creating their own scenarios... I think it will make our lives easier to be able to just give them a "load" dialog (or access it internally if you're in something like "story mode"), without having to add every scenario that you might load to index.vwf.yaml.