Update dom-text-mapper and fix the PDF plugin

 * Updated d-t-m to 654e58a2 (observer-6 branch)
 * This version of d-t-m ships with pdf-text-mapper, which previously
   lived in Annotator's PDF plugin.
 * Updated the PDF plugin
 * Followed d-t-m packaging change: load the new TextMapperCore
   script file on the demo and pages

Fixes #41.