Updated to work with new version of GitHub API as well as work on both Powershell 2.0 and 4.0.

I made some changes to get psgit to work with the updated GitHub Gist API. Additionally, I updated it to work with both Powershell 2.0 and 4.0. It should also work on Powershell 3.0 but I haven't been able to test it.