override request() to fix "429 Too Many Requests"

override request so User-Agent does not get lumped in with all urllib hits on reddit (results in "429 Too Many Requests")

http://stackoverflow.com/questions/13213048/urllib2-http-error-429
fix PR #317
Merge branch 'master' of https://github.com/davidhubbard/python-social-auth into davidhubbard-master
Make user-agent setting available for all backends. Refs #317
Merge branch 'davidhubbard-master'