Two-phased anchoring + document access modes + PDF + fuzzy pluginization

These changes include:
 * Two-phase anchoring
 * Pluggable document access strategies
 * The PDF access strategy (as a plugin)
 * Pluggable virtual anchoring strategies
 * The separation of the fuzzy matching in to a plugin
 * Various smaller changes

Since our maintenance branch is not going to be merged anywhere, but instead, we are going to separate all our changes, and submit PRs for them individually to the Annotator 2.0 branch, I won't bother separating them now.

Please review, and ask questions as necessary.

This obsoletes https://github.com/hypothesis/annotator/pull/10.