Demonstration of dataflow.
Added ruby-head and jruby-head as Travis build targets (with allow_failure).
Dataflow: make the dataflow method a member of the Concurrent module, rather than Kernel.
Dataflow: pass dependencies into dataflow blocks.
Dataflow: fix synchronisation for dependency counter.
Dataflow: make the dataflow method a member of Dataflow, and just forward it from Concurrent.
Dataflow: specs.
Dataflow: use eq instead of == in specs.
Dataflow: documentation.
Merge pull request #13 from chrisseaton/dataflow

Demonstration of dataflow.
Removed Dataflow class and put #dataflow method directly on Concurrent module.