Test reading a Gmsh file with BCs

The test reads a Gmsh file with lower-dimensional elements
which specify BCs, and solves a Laplace problem on that mesh.

Refs #1402.
Merge pull request #2992 from jwpeterson/gmsh_test

Test reading a Gmsh file with BCs (Requires libmesh update)
Merge commit '908e5e1e624f17b3fd3bc59b9780a4a5a6807dba'