Inspector layout

This is all started with a bug from the [Trello board](https://trello.com/c/LqjTxLTn/170-fix-inspector-width-issue), where the right and left panels were getting cut off, and even with `overflow` set you couldn't scroll to see all the info.

I tried fixing this "as is", and trying to see if some needed Bootstrap class was just missing in the HTML, but I wound up fighting against the Bootstrap grids to get it to look the way I thought it should (most of the trouble came from the built in gutters between columns). So I removed the Bootstrap classes from the main layout, and I think it turned out way simpler...just a few lines in a new file called `_layout.scss`. If we get into more complex layouts, I could see a grid system like Bootstrap's being useful though.

The one thing I don't like is that I put a `min-width` on the `tree-group` stuff. It didn't really work well at smaller widths, and this is quick fix to make it behave. I think it could be improved with a better solution, but it would be a bit too involved and deserves a separate branch and review.

![marionette inspector pr2](https://cloud.githubusercontent.com/assets/1902312/5209575/ec1da920-7585-11e4-84db-ab9f52a31967.png)