[In development] Add simple builder

[mdoucet]

This is not for immediate merging. Work in progress.

Current issues:

1. Parameter list doesn't update when adding a layer (which is done server-side at the moment). The model seems to update correctly (R(q) is updated).
2. Need a button to remove a layer. Removing a layer works if you set its order number to -1.
3. Need to pull in the css to make it pretty.
4. Changing the name of a parameter in the model builder doesn't work.

This PR needs the following PR for bumps:
bumps/bumps#130