Fix activity stream rendering bug

This commit fixes an issue in the activity stream module where
non-cached activities could be rendered before the AJAX requests for its
constituent parts had resolved. The reason this ocurred was that all
instances of the activity model referenced the same deferred in memory,
`this.prototype.dfd`, so once the first activity model resolved, all other
activities would be forced to render as well.
Merge pull request #24 from natgeo/feature/MIE-1525

Fix activity stream rendering bug