Make trigger pass in the old value for Moose compatibility.

Moose docs say...

           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.

The XS code could probably be written better without the has_old_value sentinel,
but I don't know how.

I pulled in t/attributes/attribute_triggers.t from the latest Moose as it has
compatible trigger tests.  They switched from Test::Exception to Test::Fatal
so I've added Test::Fatal as an include.
Merge pull request #6 from schwern/feature/trigger_third_arg

Make trigger pass in the old value for Moose compatibility.