Fixes to i18n for en->en_us and addition of common.soy.
Addition of bird.
i18n/xliff_to_json.py now removes .xlf input file.

R=neil.fraser@gmail.com

Review URL: https://codereview.appspot.com/13324044

git-svn-id: http://blockly.googlecode.com/svn/trunk@1297 c400ca83-b69d-9dd7-9705-49c6b8615e23