Two updates to the powershell build nuget package script

I'm not sure if there are any powershell experts who want to review this, but I thought I should go through the process anyway. I think that this nuget packageing script is better

Two updates to the powershell build nuget package script:

1) We can get the last package version from nuget with "nuget list SevenDigital.Api.Wrapper"
So we don't need to store the version no in a file (and do those annoying automated git commits to update it).
2) make some methods with "CmdletBinding() param()" for safety as per http://stackoverflow.com/questions/16506899/powershell-is-ignoring-expression-after-function-call