Improved oauth authorization name in Github

Previously only the bloom version and computer name were used to identify the authorization, which caused problems if those identifiers collided: https://github.com/ros-infrastructure/bloom/issues/306

Now the date is also used, for example:

![screen shot 2014-09-22 at 2 05 46 pm](https://cloud.githubusercontent.com/assets/100427/4364051/411fe27a-429c-11e4-9bb0-3866b9e08b3c.png)

Also I improved the message you get when you have two-factor authentication enabled (as I now do).