Revert "moving version into a single spot"

This reverts commit 027a67719414c14778c94e00fa3175841ddd2b10.
removing msbuildtasks package
adding custom build task
adding NuSpecUpdateTask to RestSharp.csproj
Moving build task into separate project

The assembly kept getting locked preventing me from rebuilding. Now when you run package.cmd, it will run the custom MSBuild task and create a computed nuspec file