Initial support for collection nodes

Hi,

EDIT: I've redone my patches, there are no changes to wokkel, all node-configuration is parsed with nodeOptions in idavoll.

There's a new nodes column, "collection", which represents the collection which the node is associated with (the node_id of the parent in the tree hierarchy).
Theres an initial (fake) root node with node_id=0 and collection=NULL, and all other real root nodes (leaf or collection) have by default parent collection=0 (the fake one). This simulates a multiple-root hierarchy with a single root in the nodes table, which makes the queries easier.

PubSubResource's getNodes() no longer ignores the nodeIdentifier parameter; this is passed to the backend (and the psql storage engine), and all direct child nodes of nodeIdentifier are returned. If there are no collection nodes, getNodes() gets an empty nodeIdentifier (''), and all nodes are returned (because all existing nodes are root nodes).

So creating and configuring collection-nodes (and leaf-nodes) is now possible.