Added an optional build script flag to target the .NET Framework 4.5

I was able to get ConfuserEx to run just fine with Mono on OS/X.  The only trouble I ran into was that the TaskParallelLibrary back-port is not compatible with Mono (seems it uses Windows-only native libraries under the hood).  By changing the target framework to 4.5 and removing the reference to TaskParallelLibrary (since it is unnecessary when targeting .NET 4.5) I was able to get it to work.

.NET 4.0+ also includes its own (immutable) Tuple types so I excluded the Tuples.cs file from the build.  This also required changes in a few places where the .Item properties of tuples were set directly so that the code works with both framework versions.

Finally, the "extern alias PTL" workaround is unnecessary for .NET 4.5 so I excluded that as well.

The framework target is set by an optional command-line parameter to Build.cmd:

    Build.cmd NET45

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