Remove Invoke-Build NuGet dependency and replace with bundled Invoke-Build.ps1 script

[lholman]

To make OneBuild a little smaller on the dependency front, and also avoid the chicken and egg situation where we can't start a OneBuild build until Invoke-Build is NuGet package restored let's bundle the latest Invoke-Build.ps1 with OneBuild.
This was kind of mentioned by @nightroman in the final comments of #47 too