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 it in a file (and do those annoying automated git commits).
2) make some methods with "CmdletBinding() param()" for safety as per http://stackoverflow.com/questions/16506899/powershell-is-ignoring-expression-after-function-call
Merge pull request #89 from AnthonySteele/powershellupdate

Two updates to the powershell build nuget package script