[Dbix-class] have add_to_$rel return $link for many to many

[Peter Rabbitson]

Todd W wrote:
> Hello,
> 
> I'd like add_to_$rel return the $link instead of $obj in
> ::ManyToMany.pm.

This can very well break code that relied on the previous behavior.
Please detail your reasons for doing this, and wait for others to
weigh on whether the change is worth it.

 I've inlined a diff below that does so, with an updated
> test and documentation patch against
> http://dev.catalyst.perl.org/repos/bast/DBIx-Class/0.08/tags/0.08109

Patches normally go against 0.08/trunk, but that'll do too.