Safer task handling.

Safer task handling.
Log message

Added a warning for unsupported units.
Exploiting StandardUnit

Exploting SDK's StandardUnit for string conversion and checking
supported unit.
Bug: config set unit

Unit set in config in conjunction with overriding pattern unit bugged
out.
Added support for StandardUnit in config but...

Added support for StandardUnit in config but due to a limitation in
log4net, this doesn't quite work. Anyway, units are now internally
stored as StandardUnit in both appenders.
Reverted sample app.config.