Adding MaterialPointSource and associated test.

It is a DiracKernel test that uses a MaterialProperty, as has been
implemented by an earlier patch on this branch.

If uncommented, the print statement in
MaterialPointSource::computeQpResidual() should show that the
MaterialProperty is being correctly evaluated at the DiracKernel
point.

Refs #2379.
Adding an expect_err test which demonstrates attempting to use a
stateful material property with a DiracKernel.

Refs #2379.
Slightly more robust error checking in SolidModel to avoid dereferencing a NULL pointer.

Refs #2379.
Regolding Richards Dirac bh07 test.