Refactor parser creation out of parse_ots_args

[hstock]

Create a function which creates the parser and use it in parse_ots_args. parse_ots_args
should only call the parser.parse method and verify the options.

This would help with creating automatic documentation, also see #101 .

If desired, I can supply a patch.

[petertodd]

Sure, that's fine. It can be part of your #103 pull-req; doesn't need to be a separate pull-req.