Add and test MechanicalContactConstraint ref #2816

MechanicalContactConstraint is a general class for doing the full
set of contact formulations and friction models currently supported
by the DiracKernel-based mechanical contact system.

I've added a few 2D tests of this capability with this commit.  We're
working on a more comprehensive contact test suite.
Merge pull request #3020 from bwspenc/contact

Add and test MechanicalContactConstraint ref #2816