Fixing authentication with yml file

[robustdj]

This patch fixes authenticating through a yml file. It can now be done like this:

authenticated :config => "github.yml" do (...) end

Before the .gitconfig contents were always overriding the yml file.

[radar]

Hello, firstly: thank you very much for the patch. I have a couple of comments though, and I will add them now.

[robustdj]

let me know if you think my pull request needs anything else

[radar]

Nope, looks good. I'll merge now.