Added new test for blocks which provide output on different channels for different iterations.
Added "time" as a field in the channel class, rather than just assuming that channel time is identical
to the time of the data it holds (channel time can advance while the data stays the same).
Added lists of "visited blocks" to the tests.
* Additional tests
* Fixed bug that arises in certain graph topologies when the blocks provide only a subset of the possible
  outputs on a given iteration.
* Updated setup.py to obtain its "long_description" from README.md.
* Added CHANGELOG.
* Added dev_requirements.txt (use 'pip install -r dev_requirements.txt' if you wish to have Conduit installed
  along with its requirements for developers (e.g. the nose testing framework)