Add Poisson solver to ghost mesh demo.
Get assembly working for ghost meshes.

Need check in assembly loop for ghosts. Will go when iterators are extended.
First DG example running in parallel.
Some paralell dg debugging.
Merge remote-tracking branch 'origin/chris/ghost-mesh' into garth/ghost-mesh-dof-map

Conflicts:
	demo/undocumented/ghost-mesh/python/demo-ghostmesh.py
	dolfin/mesh/MeshEntityIteratorBase.h
Work on dg elements in parallel
Get IP DG working in parallel.
Get dofmap builder working for node- and facet-wise ghosted meshes.

Ghost mesh in IPDG demo - now runs in parallel.
Merge branch 'garth/ghost-mesh-dof-map' into chris/ghost-mesh

Conflicts:
	demo/undocumented/ghost-mesh/python/demo-ghostmesh.py