suppress plus1 link on nodes where plus1 is disabled

howdy!

plus1 has a setting named plus1_nodetypes that lets the administrator select which content types should show the plus1 voting widget

however, plus1_links does not respect this setting - so if you don't allow voting on a content type, you still see "0 Votes" or whatever on every node for that content type. I don't think this makes sense.

Here's a patch for D6. I can make a D7 patch as well if maintainers are interested.