ENH/BUG: ignore line comments in CSV files GH2685

closes #2685

I have added the ability for both the C and Python CSV parsers to ignore commented lines (i.e., lines beginning with a comment character).  Currently the C parser preserves commented lines as empty lines (all NaN), while the Python parser ignores them all together.

In addition, I fixed a small related problem with the CSV format sniffer in the Python parser.

I plan to finish up this work by ignoring empty lines as per #4466.