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).

.gitignore

@@ -1,5 +1,4 @@
-iD.js
-iD.min.js
-iD.css
-build
+dist/iD.js
+dist/iD.min.js
+dist/iD.css
 transifex.auth