refactor contributor list generation code into an attribute builder

The contributor list is used in a couple places and stored in an attribute, so
we may as well make the attribute responsible for building it, as well.

Note that this is still a builder method, not a default.  See
MooseX::AttributeShortcuts (at least 0.15).
v0.004

	* Add the list of contributors to META.* (gh#1, thanks dagolden!)