Add "unique id" caching for Dirac points

This branch adds an interface for specifying unique IDs for Dirac points when calling addPoint().  It also adds the capability to ask the DiracKernel what the cached ID for the current point is within computeQpResidual/Jacobian() routines.  Tests for the new capability have been added to MOOSE, and two of @WilkAndy's DiracKernel classes have been modified to use the new interface.

Refs #2364.