Add Twitter integration

[sberrevoets]

For easy tweeting, it would be nice to be able to log in to Twitter and automatically tweet when the user files a new radar. Would be nice to be able to provide a format string and to give the user the option to not tweet when the radar was filed as a dupe.

[sberrevoets]

Whoops, just noticed #51

[keith]

I think a pre-req to this is figuring out an interface for easily switching on and off the services you want to post to. There's an example of one solution to this from #45

Another possible pre-req to this, depending on how we want to do this, is #49 because if we want to allow disabling of Apple Radar (say because you posted to Apple Radar and forgot to post to Open Radar, so now you need to backfill to only Open Radar), we would still need the Radar ID.