User shouldn't specify their GitHub username

[GoogleCodeExporter]

The issue exporter script takes both a github user token and a username, when 
the username can be derived from the token by looking up the current user's 
information (https://developer.github.com/v3/users/#get-the-authenticated-user)

This would leave one less option for the user to specify and would simplify 
logic to guarantee that the current user is the user they claim to be.

Original issue reported on code.google.com by [email protected] on 4 Nov 2014 at 4:16