Support use of UnionArgParser in .NET 3.5 projects.

* Add new project file targeting .NET 3.5 and containing same files as 4.5 proj.
* Update build script to fail if proj files are not in sync.
* Change Guid.Parse usage to ctor parse (as former .NET 4.5 only).
* Update nuspec to include net35 binaries.
* Add helper fsx that can be run to sync projects.  Note: file ordering is not correct in FAKE yet so may need to tweak.  [Raised FAKE issue here](https://github.com/fsharp/FAKE/issues/454).