Store annotations relative to content, not content display

 - add nid field and index to database
 - allow query by nid through store API
 - configure store plugin to query by nid and ignore URI
 - create a plugin for node-based annotation (annotator_drupalnode.js)

DESIGN

We store the appropriate node id (nid) with every annotation. We override the
annotation API to search by nid rather than URL. We also manipulate the url to
be always the path to the node detail page.

SETUP

 - in admin/config/content/annotator, enable the DrupalNode plugin
 - in admin/config/content/annotator, set Annotator element to ".node :has(>p)"
   [this selects the div immediately surrounding the node content. Any other selector should
   also be fine, provided the markup within it will be identical on both node and index pages]
 - there is a new field in the annotation module. So you will need to either reinstall, or
   manually add the nid field
I think that is better to use a check_plain() on this data.