Add a test_dirichlet_nan.sh regression

This hits the parsed_dirichlet BC.  It depends on the dirichlet_nan
and fparser_nan features (one just pulled, one hopefully soon to be)
in libMesh 1.0.0-pre.
Merge pull request #116 from grinsfem/test_dirichlet_nan

Add a test_dirichlet_nan.sh regression