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.

This screenshot is from a `0.9.x` instance. The only difference will be the addition of `share.png` for the "Short URL" button. 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 34 19 pm](https://cloud.githubusercontent.com/assets/494338/4111374/c5ca2d24-3206-11e4-8dcf-7b95005799ee.png)