Compute Material Properties at DiracKernel points

This branch adds the ability to evaluate and use material properties at the points of DiracKernels, which was not supported before.  Stateful material properties are currently not supported for DiracKernels, however.  If you try to access one from a DiracKernel, MOOSE will throw an error.

@WilkAndy, this branch causes two of your tests
```
richards:dirac.test_bh07
richards:gravity_head_2.test_gh02
```
to diff for me.  This may due to them not using correct MaterialProperty values before, so perhaps those tests could just be re-golded.  Let me know if you have any questions.

Refs #2379.