Big API change + support for multi-page document access

API changes:
 * NEW: getCorpus() returns the corpus
 * NEW: _updateMap() updates the mappings
 * NEW: _testMap() does some consistency checking
 * NEW: stub methods for simulating page-based, lazy rendered environment.
   (Used by two-phased anchoring in Annotator.)
   * getPageIndex()
   * getPageCount()
   * getPageIndexForPos()
   * isPageMapped()
 * CHANGED: constructor now accepts a text ID, used for logging
 * CHANGED: getMappingsForCharRange() now accepts a list of page indexes
   to look at, and also returns the results on a per-page basis
 * CHANGED: the static @changed() method now accepts arbitrary data
 * CHANGED: getInfoForNode() now returns page number, too
 * REMOVED: postProcess() hook
 * REMOVED: getDocLength() (use getCorpus())
 * REMOVED: getContentForCharRange (use getCorpus())

Changed behavior:
 * The change notification events now set the lastDOMChange
   and lastScanned flags, to avoid unnecessary re-scannings.
   The drawback is that we might miss independent changes,
   but those are not yet handled properly anyway.

Internal changes:
 * Changed internal logging
 * Many cosmetic changes and debugging improvements
 * Small internal simplifications