Beam backend is missing some pipes

[johnynek]

[warn] /Users/oscar/code/oss/scalding/scalding-beam/src/main/scala/com/twitter/scalding/beam_backend/BeamBackend.scala:29:27: match may not be exhaustive.
[warn] It would fail on the following inputs: (CounterPipe(_), _), (CrossPipe(_, _), _), (CrossValue(_, _), _), (DebugPipe(_), _), (EmptyTypedPipe, _), (ForceToDisk(_), _), (TrappedPipe(_, _), _), (WithOnComplete(_, _), _)
[warn]       def toFunction[A] = {
[warn]                           ^
[warn] one warning found

Those need to be added to otherwise you will have errors at runtime. I think all of them are pretty simple to implement.