Nagaconda

- Added 'set_status_message' method to enhance human readability in generated
output beyond OK, WARNING, or CRITICAL.
- Added 'unknown_error' method to give plugin abortive quit capability and
avoid polluting 'finish' with similar functionality.
- All internal assertions now exit gracefully with 'unknown_error'.

- Fixed a problem that was breaking the lower-bound for negative infinity
in warning and critical thresholds.
- Added 'default' to option attributes passed to OptionParser.

- Initial Release