Complete conversion to using mixins for icons

Experimentation shows that this is not helpful for people to redefine
what the icons look like. The separate loading of the `styles.less` file
strikes again! It looks like the only thing that will solve this is
fixing Atom to decorate files with the scope for the file :cry:
Shift the alignment and size of DevIcons icons

The alignment and size of the DevIcons icons is not as consistent as
either FontAwesome or Octicons. I've tested these new values out on
the Grunt and JS icons and they look to be pretty good.

Fixes #10