Create a dist directory

This directory will contain the built sources (iD.js, iD.css) and images. On a release branch, we will check in the generated files. For the osm.us/iD/master/ deploy, it should run make and use dist/ as the root.

Paths in css are relative to the iD.css file in the dist root, e.g. url(img/foo.png).

@iandees, this is mainly a heads-up for the osm.us master deploy. I'll wait until you have a chance to get it reconfigured for this change before merging.