Add support for the fury --as option

Details:
http://devcenter.gemfury.com/articles/collaboration.html#impersonation
Add details of the :as option to README
Fix pushing to gemfury

It needs the arguemnts in a particular order when called through Open3
for some reason.
Merge pull request #4 from alphagov/support_gemfury_as_option

Support pushing to different gemfury account