Update to latest version of simplejson.
Update to latest version of nose.
Clarify api usage policy.

Remove the non-commercial/open-source aspect, as the community noted this
as problematic.
Add a downloads page, listing all the files in the export/ bucket.
Add a mocked functional test for listing downloads.
Cache the S3 download list lookup in Redis for 10 minutes.
Add changelog and enable cell export tasks, closes #295.