Add a uri_scheme option

This allows github enterprise installations that don't use https to use the
hub gem.

I think all installations should use https, but I don't control my company's
install, and their argument for not using https is that the install is only
available over VPN, so traffic is already encrypted, and using https means
they'd need to buy another https cert.

So until I can convince them otherwise, I'd still like to be able to make
pull-requests from the command line.

The cucumber tests don't run for me

    undefined method `posix_spawn=' for ChildProcess:Module (NoMethodError)

so I haven't updated them.  If I comment out that line in
features/support/env.rb there's already failures in the master branch, so it's
hard to tell if my changes could have broken anything.