Added i18n documentation and starting places for Stanford hackathon 

Changed:

1. I extracted i18n testing info from TESTBED.md and created an I18N.md doc with that info plus more helpful stuff.

2. I broke out the functionality of `update_po.py` into individual parts that can be called by using the different options. I also added a function stub for the people participating in the hackathon to have somewhere to start. 

@bcipolli you're pretty familiar with this code already from the number of times you had to test it for me... 
To test, you can run `python manage.py update_po -h` to see the options, and run with each flag to test functionality. 

@wangguan59 if you have time, I would really appreciate someone with a UX eye reading through the docs and seeing 1) if they make sense and 2) if following the instructions gets you up and running from scratch.