Remove MPIContext.BASE_COMM, remove self.intercomm refs.

BASE_COMM wasn't necessary after all, just a single INTERCOMM is
sufficient.
Merge pull request #545 from enthought/bugfix/intercomm-refs

Remove MPIContext.BASE_COMM, remove self.intercomm refs.
Adds Context() factory function.

Takes 'kind' argument to allow user to select kind of context, either
'MPI' or 'IPython'.
Merge pull request #550 from enthought/feature/context-factory

Adds Context() factory function.
Refactoring of test base classes.
More refactoring and testing of test base classes.
Merge pull request #551 from enthought/bugfix/contexts-and-testing

Fix bugs with Contexts and Test base classes
Support running MPI client from single entry point.
Merge pull request #552 from enthought/feature/mpi-client-one-script

Support running MPI client from single entry point.
Merge branch 'master' into feature/julia-example
Simplify mpi_benchmark.