Add Venmo button

Four months after saying a Venmo pay button was "one to two weeks away" in https://github.com/gittip/www.gittip.com/issues/1922, we've finally gotten around to soft-launching our official button! I'd love for Gittip be one of its first users, since I think it's a much better flow than just linking to a user's profile.

This PR adds the Venmo button to the Venmo row in connected accounts:

![screen shot 2014-05-09 at 4 53 02 pm](https://cloud.githubusercontent.com/assets/579628/2933204/ee8b0652-d7bb-11e3-9e38-8d463560ffb7.png)

You can see the flow in action here (it's undergone a couple of visual tweaks since this):

http://quick.as/6m6s62j

This also fixes #2361.