[bug 984868] Fix ./manage.py extract

* add missing translate-toolkit library
* update tower to latest version which works with Babel 1.3 which
  provides an additional context parameter
[bug 927888] Add translation_system field

* add translation_system field to Product table and default it to null;
  there aren't any translation systems, so this isn't a particularly
  interesting change
* add migration
* add column to products table
* fix notes field so you can have products with no notes
[bug 927888] Add translation infrastructure

* create translations app
* create register_auto_translation class decorator
* create celery task for translating fields in model instances
* create translate utility function
* create translate_handler post_save handler
* create translation "plugin" system for centralizing system-specific
  code
* create fake translator
* create dennis translator (because hey--why not?)
* create translation_sync command
* write initial tests
[bug 927888] Hook up translations to Response