VBENCH: rename parser.py to parser_vb.py
closes #5972

@dsm054, does this resolve the issue for you?

Yep, seems to, and I went through a full test_perf.py to make sure that the parser_vb benchmarks were still being executed.
 Worked for me too. Strange that it's not fully reproducible, I haven't had this
problem. Can you report versions? would like to understand where the difference lies.
 @dsm054 are you on a Mac with homebrew python?
 @TomAugspurger: nope.  Had the problem on both my notebook (ubuntu 13.04) and my work box (12.04).
