Added documentation for TAP_LOG_SHOW_USAGE
Bumped dist version
Added debug level 'all' for suppressing all logging
Improved handling of nonsense log level filters

Previously, an invalid filter would throw an exception.  This means
the whole program was likely to crash from a potential typo in an
environment variable.  I decided it was better to warn and continue
running.

Also, I now handle level+N that exceeds the max level by silently
capping it at the max level.

Also fixed a bug where level+N wasn't being calculated correctly.
Warn about exceptions during debug/trace

Debug and trace methods trap exceptions, but were doing it silently.
I now generate a warning when it happens.

Also I wasn't catching errors in the printf versions of debug or trace,
and am now.
Updated all documentation
Added testcase for filter+N notation
Updated Changes log
Minor doc tweaks
Dev version 0.2.0_02