Create hook for access control

I needed to be able to add additional conditions to determine whether voting should be allowed or not for a node - and I of course didn't like the idea of having to hack Plus1 to accomplish that.

I suggest adding a new hook, similar in design to hook_node_access(), that is used to determine when to show a widget and when to allow a vote.

That makes it possible for me to add extra conditions in my own module - I can for example say that all nodes not tagged with "Voting" shouldn't be voteable or say that all nodes contained added to a group should inherit a vote-setting from that group etc.

Attached is a patch that adds such a hook - "hook_plus1_access()".