More attribute options

Accessors can automatically dereference ArrayRef and HashRef type constraints.

my $foo = Foo->new(bar => [ 1, 2, 3 ]);

foreach my $blah (@{$obj->bar}) { ... } # yuk!

has 'bar' => (..., isa => 'ArrayRef', auto_deref => 1, ...);

foreach my $blah ($foo->bar) { ... } # horray! 

Available for HashRef as well