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/master deploy, it should run make and use
dist/ as the root.

Paths in css are relative, e.g. url(img/foo.png).
Update build/install directions
Clean up Makefile
Merge branch 'dist'