Replace old composer toolbar icons

This removes a bunch of crufty icons that haven't been used in years, and also replaces the patchwork toolbar icons for Composer with a matching set of flat icons adapted from the [Font Awesome](http://fortawesome.github.io/Font-Awesome/) project.

**Updated** screenshot to show the "Short URL" button. This is how everything would appear for `master`. If this looks :+1: to everyone, I'll submit a separate PR for `0.9.x` as well.
![screen shot 2014-09-01 at 2 43 40 pm](https://cloud.githubusercontent.com/assets/494338/4111412/f0214124-3207-11e4-9262-c556412e3a75.png)