High order 2294

OK, this is my first pull request. I am adding the high order elemental auxiliary variable. It includes modifications in MooseVariable and AuxKernel. AddAuxVariableAction has to be changed accordingly. Quadrature order has to be set with AuxilarySystem. Note that we donot need to change quadrature order for nodal auxiliary variables (otherwise one moose test, test/tests/bcs/nodal_normals/cylinder_hexes_2nd.i, will fail). A simple test is added. Four postprocessors including two L2 norms are added to make sure the high-order variable in the exodus output are indeed different with high orders. Ref. #2294