$computed w/ "resources", exception fixes

Functions passed to $computed can now have an optional .destroy() method
to allow them to "clean up" resources when they're no longer needed (ie.
when the $computed is deregistered).

Exceptions now have well defined handling:
 - an exception in a $computed function is passed to $exceptionHandler
 - the result of the function is treated as `undefined`
Bump the version number to 0.0.2