Celluloid::Properties

A "property" macro for adding declarative properties to a class which are
inherited but overridable in subclasses. Sort of like what class variables
always should've been.

Just an idea for now, but this can be used to replace things like
"mailbox_class" and "execute_block_on_receiver"
Use Celluloid::Properties for all actor class properties

This results in some minor API changes, specifically mailbox.max_size as a class
attribute, which is replaced in this commit as mailbox_size

I know there are plans to change these, but this largely preserves the existing
API with a few minor changes.