Distributed server management command to download, unpack language packs, and update database to reflect newly installed languages

Addresses #852 and depends on PR #844 "Beginnings of i18n"

Functionality: Command downloads language pack from central server, unpacks it into correct locale directory, and updates database based on metadata file included with the language pack. 

Testing: Create a test environment, with branch i18n_ui set as central server, and this branch as distributed. Run cache_translations on the central server. Then run the server. On distributed server run manage.py languagepackdownload -l <your_language_code>

Expected behavior: it will download the pack you specify, if it exists, and unpack it into the locale directory. It will error out when it comes to updating the database, because I haven't committed a migration yet (want to first confirm with @jamalex and @bcipolli that the LanguagePack model has all the field we want and need. This was tacked on to the final commits of #844. 

An example of KA Lite in Arabic (to get people excited/was generated in <1 min)

![screen shot 2013-10-18 at 3 13 17 pm](https://f.cloud.github.com/assets/1319950/1364642/b841b758-3842-11e3-8514-385710da2c01.png)

:clap: :+1: :smile: