Add node field condition to CA

The attached patch adds a condition to CA that allows you to compare a node field value. For now it simply works with basic node fields and simple operators. It worked in the client site I tested it on, though. Looking for more eyes. : )

This patch depends on the node entity defined in Lyle's patch here. You don't need to apply the whole patch if you don't have it... you can just grab the node entity code out of it and put it into ca_ca_entity() in ca.ca.inc.