Implement attribute triggers

[schwern]

From Moose...

       trigger => $code
           The trigger option is a CODE reference which will be called
           after the value of the attribute is set. The CODE ref is passed
           the instance itself, the updated value, and the original value
           if the attribute was already set.

           You can have a trigger on a read-only attribute.

           NOTE: Triggers will only fire when you assign to the attribute,
           either in the constructor, or using the writer. Default and
           built values will not cause the trigger to be fired.

[tobyink]

I've moved the Mite issue tracker here.

This is now implemented.