Fix issue 155: BoundaryMesh generates a mesh with too few vertices under certain conditions

Sort discrepancies of shared vertices between Mesh and its BoundaryMesh
Merge branch 'oyvinev/fix-issue-155'
tiny fixes in Cahn-Hilliard documentation
Merged in blechta/fix-CH-doc (pull request #68)

tiny fixes in Cahn-Hilliard documentation