add python exception for ReadParser with threads=0

### fixed issue #86:
- add python exception when ReadParser with threads=0;
- add  test_with_zero_threads() function in tests/test_read_parsers.py;
I don't think this is a very useful error message. "Invalid number of threads, must be greater than zero." would be better.

Also, we should test for this around line 650 instead of waiting for it to blow up :-)