Better reporting and other minor enhancements

User2userpoints is a useful module to allow users to donate their own points to other users. I've added some features and made some structural changes to allow for better reporting and other niceties:

* Allow to pass a node id on the user2userpoints URL. This node id is then stored as the entity_id in the userpoints_txn table, along with entity_type = 'node' and operation = 'user2user'. This allows to find out how much each node generated as donated points, and can be useful as a ranking mechanism. The function user2userpoints_nodepoints($node) calculates that sum of donated points. As a side benefit, the user2user operations are now distinguishable from other userpoint transactions.

* Redirect to the node page if the node id was passed to the user2user form.

* Set the title to 'Give !points' or 'Give !points for %nodetitle' instead of no title at all.

* Add a setting to choose which node types should display the 'Give !points' link.

Thanks for your consideration.