Add support for nuget pack -Properties switch
Add support for nuget pack -Properties switch

One of the most handy things this does is allow you to specify the build configuration:
  nuget pack foo.nuspec -Properties Configuration=Release

wrozmiarek/msbuildtasks,stimpy77/msbuildtasks,AArnott/msbuildtasks,loresoft/msbuildtasks,smallkid/msbuildtasks,arekbee/msbuildtasks,jricke/msbuildtasks