Xfail serialization test for now.
Removed serialization test files from repo.
Update .gitignore
Miscellaneous cleanup.

* Added docstrings to some of the functions.

* Changed all the print statements to log.debug statements, so they
only print when debug logging is set to true.

* Made the Simulator have the same call signature as the refimpl nengo
Simulator. Required to pass some of the tests.

* Fixed bug caused by the fact that some Probes add a conection when
they are built. Such connections were not being handled correctly by the
partitioning code.
Improved ElementwiseInc operator.

So that it operates under the same broadcasting rules as the refimpl
ElementwiseInc.
Improved how probe data is returned.

The refimpl always uses vectors where possible (i.e. if one of the
dimensions of the signal is 1). The commit changes the behaviour of MPI
to match this.
More complete probe printing.
Added some printing functions.