Bug 903149 - Part 1: Add jsmin Python package; r=glandium

Exported source code from changeset fdfa1b187681 from
https://bitbucket.org/dcs/jsmin without modifications.
Bug 903149 - Part 2: Add jsmin to virtualenv; r=glandium
Bug 903149 - Part 3: Support for minifying packaged JavaScript; r=glandium
Bug 903149 - Minify the chrome js code: enable by default on b2g r=glandium