reddit sometimes responds with "429 Too Many Requests" seemingly randomly

http://stackoverflow.com/questions/13213048/urllib2-http-error-429 fixes the problem

Basically reddit rate-limits their API based on the User-Agent string. Instead of sticking with the default (urllib User-Agent) this pull request sets User-Agent to 'python-social-auth-' + __version__ so requests shouldn't hit the rate limit without knowing it