Rename Gnuplot outputter
closes #3434
Merge pull request #3437 from permcody/gnuplot_rename

Rename Gnuplot outputter
Merge pull request #3442 from jwpeterson/fixup_dirac_offdiag

Fix for Dirac off-diagonal Jacobian entries.
Only compute off-diagonal Jacobian entries if variable has DOFs.

This patch does the proper checking at the
ComputeDiracThread::onElement() level rather than the
DiracKernel::computeOffDiagJacobian() level, and undoes the quick fix
I committed earlier.

Refs #668.