fix for --disable-optional
For --disable-mpi, we define COMM_WORLD as an int in libmesh_common.h, where for chicken-and-egg reasons it is difficult to include all of parallel.h.  Correspondingly, later things freak when trying to construct a Parallel::communicator from an int, as that constructor is not defined.  This implements that constructor.
Merge pull request #83 from benkirk/fix_nompi

Fix nompi configuration