Try to serialize requests to update the blog feed

Make a best effort to only allow one client/request to make the backend
HTTP request to the blog feed.

Addresses issue 928 (http://openhatch.org/bugs/issue928)
Merge branch 'master' of https://github.com/bobveznat/oh-mainline into bobveznat-master

Conflicts resolved:
	mysite/customs/tests.py