optional communicator to mesh constructor.  defaults to CommWorld, not used ATM.
make beautify
begin using per-object communicators in mesh classes
libMesh::ParalellObject base class for all things that require their own communicator reference.
derive key data structures from ParallelObject
refactor DofMap as a ParallelObject
refactor BoundaryInfo as a ParallelObject
refactor to use per-object decomp info
refactor ExodusIO/NemesisIO as ParallelObjects
refactor XdrIO as a ParallelObject
refactor ExodusIO/NemesisIO as ParallelObjects
refactor MeshBase as a ParallelObject
refactor ParallelMesh as a ParallelObject
refactor MeshCommunication with ParallelObject, and fallout
refactor partitioning
refactor MeshTools
clarify comment
refactor MeshRefinement as a ParallelObject
refactor MeshRefinement as a ParallelObject
refactor misc mesh-related classes as ParallelObjects