Allow OAuth authentication dance with no local webserver

I'd like to be able to do the OAuth2 dance with no local webserver at all, i.e. by manually copy-pasting a URL and then an authorization code. This is clearly less exciting as a user experience, but is sometimes a necessity (I often find myself doing the auth dance on a machine which isn't visible to the outside world, or only has SSH access). 

I've actually got a version of this I've been using locally, and I'll submit a pull request shortly. In particular, I added an extra `use_local_webserver` argument to `oauth2.0_token`, though I'm happy to thread that out as far as is reasonable. I'm also happy for someone to come up with a better name for the flag.

This is the same request that was mentioned in #31, but is not quite the same as the solution proposed by #32, unless httpuv is cleverer than I think. ;)