[Summon 2] Cleanup, add support for individual pages

Detection on multiples should be broken now, because it requires monitorDOMChanges, which makes the page unusable right now.
Test doesn't actually work on the "book" item, for unknown reasons. Possibly because offsetHeight is not set in a hidden iframe? (making this up)

Please do take this for a spin. I'd say enable the commented out `monitorDOMChanges`, which makes detection work very well while navigating the page, but it really slows down to a halt fast.
Right, the individual results took me a while to find, but I knew I had come across them before. This patch should cover them. I'll take a look at the Chrome vs Firefox issue when I get a chance.

If you can, I would suggest running Firefox with Zotero from github including changes in https://github.com/zotero/zotero/pull/592 This way you can uncomment the `Z.monitorDOMChanges` part and see how it works. From my testing in Firefox it's working quite well. I haven't tested in Chrome though. OTOH, I imagine that the URL bar icon doesn't really matter to you or PME.