Two-phase anchoring + support for PDF + refactoring

 * Implemented two-phase anchoring, for supporting PDF and other
   lazy-rendering, page-based systems
 * Decoupled the -mapper and -matcher libraries.
   (See https://github.com/hypothesis/dom-text-mapper/commit/a6cee76db6a5807495155a0cb299ff393d86c0ae and https://github.com/hypothesis/dom-text-matcher/commit/5d954e4d1d4cb83fbe921ff8144fe3238b1233a9 for more explanation)
 * Factor the -matcher initialization out of the strategies,
   since it's indepentend now.
 * Add support for dynamically registering document access strategies
 * Implement PDF.js-based document access strategy in a new plugin
 * Add support for dynamically registering virtual anchoring strategies.
 * Break out the fuzzy anchoring strategies to a plugin,
   and add to [demo|dev].html
 * Updated dom-text-mapper to fcd26632 (master branch)
 * Updated tom-text-matcher to bf0f0224 (master branch)

This obsoletes #11, and incorporates all the review comments from there.