Fixed WTS to emit separators even when HTML has modified content.

* When WTS cannot emit separators from source wikitext, it emits
  normalized separators from node handlers.  Since node handlers
  run before the separator handler runs, WTS briefly buffers the
  normalized separators and emits them or discards them when the
  separator handler runs.
* Updated documentation of state object.
* Changed parserTests.js to set default value for 'use_source' to
  false.
* No change in parser test runs.
* This patch now completes the refactoring and rewrite of the
  serializer to a more saner structure -- pending the inevitable
  bug fixes.

Change-Id: Id24652206d43b312514b0bc124e26b9604d130e3