ExceptionHandler enhancement implemented in the exceptionmagic branch

[Stephen Weeks]

The exceptionmagic branch now holds an implementation of filtering
exceptions in ExceptionHandler's can_handle.  If I could get some API
feedback, that would be great.

Relevant commits are:
http://www.parrotvm.org/svn/parrot/revision?rev=31331
http://www.parrotvm.org/svn/parrot/revision?rev=31332
http://www.parrotvm.org/svn/parrot/revision?rev=31355