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).

test/img

@@ -1 +1 @@
-../img
+../dist/img/

test/index_packaged.html

@@ -17,7 +17,7 @@
     <script src="lib/bind-shim.js"></script>
 
     <!-- include source files here... -->
-    <script src='../iD.min.js'></script>
+    <script src='../dist/iD.min.js'></script>
 
     <script src="spec/spec_helpers.js"></script>