Alphabetical tree-view sorting (files and directories mixed together, Mac OS X finder style)

An experimental implementation for addressing #64, and perhaps a path towards addressing #235.

I thought I would open the conversation about this approach before I take the time to:

- [ ] add a configuration parameter alphabetical sorting
- [ ] create specs
- [ ] search for odd edge cases
- [ ] add implementation for case sensitive sorting

Here's a preview of what it looks like on a typical Rails project.

![screenshot 2014-11-23 21 59 19](https://cloud.githubusercontent.com/assets/21340/5160518/11e0bff2-735c-11e4-83f8-cbc860cbfe14.png)