Release/0.5.2rc2

Takes @egonina 's work and ads the final few bits to get green tests. Given its for the forth coming scalding release it seems good to just merge into develop with changes this much here.

@johnynek changes here are all around the scalding platform, could you take a look? Thanks!
WritableSequenceFile should be a TypedSource[(K, V)]:
https://github.com/twitter/scalding/blob/develop/scalding-core/src/main/scala/com/twitter/scalding/WritableSequenceFile.scala#L46

So, we should be able to write:
val src = WriteabseSequenceFile[K, V](...)
TypedPipe.from(src)