Fix path in demos Makefile
Fix gitignore for new path structure
Adapt to changes in PyOP2 matrix assembly

The Mat user must now call assemble explicitly to ensure the matrix is
built before using it.
Allow exterior facet integrals in parallel
Set up fieldsplit ISes correctly for parallel running

Numbering of fields on process N is offset by the sum of the rows of the
big matrix on processes 0, ..., N-1 (inclusive).
Test mixed poisson problem in parallel

Use both normal gmres+jacobi solve and fieldsplit solve.
Merge pull request #242 from firedrakeproject//matrix_assemble

Adapt to changes in PyOP2 matrix assembly.  After queuing up par_loops
to assemble matrices, we must explicitly call matrix.assemble()

Additionally, remove restriction on assembling exterior facets in
parallel and add parallel fieldsplit support (tested with mixed poisson
problem in parallel)