Use only of PowerShell in NuGetPackageBuilder.ps1, also use inclusion list of files to add to nuget package to prevent any accidential files there

Follow up to 6e58b02bff0cd58a9358410905faa0a324818b2e
New fix for issue #87

This fixes issue introduced long time ago in d706e5290a1195fd3647e135cc21a4e9172f45a3
The problem was when psake was imported as a module in one dir, and then executed without arguments from another
Also added using current config, not config_default when detecting build file.
Also use the same default parameters in psake.ps1 as in Invoke-Psake