Added support for multiple languages

Added support for multiple languages
Moved english strings to _locales/en folder
Updated build.py script to always process _locales folder

![flag_735x325](https://f.cloud.github.com/assets/634478/1692552/7c45b8aa-5e7e-11e3-9ba7-e1fac2d1531c.jpg)


BUG=https://github.com/GoogleChrome/text-app/issues/183