Flow related transformations (sort, sortedjoin, split).

Created new transformations for doing multi input/output flow related
operations. Software sorting of a stream (not recommended unless you're certain
of the relative smallness of your stream), flux splitter (route messages on
various outputs depending on the result of a condition), sorted join (join two
streams sorted on the same key using a 'merger' function.

Fix a bug on InputMultiplexer.get() when more than one input is used, add unit
tests for it.