add: related_namespace()

Yay, sugar!
nit! rejigger some whitespace
SYNOPSIS pod init_arg correction

Note that our init_arg for the constructed class attribute (thinger_class in
this case) is set to undef, not the original class attribute.  The original
class attribute's init_arg is set to the name of the constructed class
attribute (e.g. original_thinger_class' init_arg is 'thinger_class').

This allows things to work, as expected, without worrying too much.
Expand on the namespaces doc
v0.009

	* add related_namespace() sugar
	* fix some inaccurate code in SYNOPSIS