Adds Mobiles, fleshes out VnumSection

Mobiles inherits from VnumSection. Mobiles file also contains Mobile,
the actual mob object. VnumSection class handles the splitting and
parsing of the section, while Mobile class handles the parsing of the
entries within.

That's a pattern that Rooms and Objects will follow, as they'll also
be child classes of VnumSection. (Speaking of which, VnumSection calls
self.class.child_class to determine which new object to instantiate for
each entry it splits off. Mobiles.child_class returns Mobile, for ex.)