Fixed serializer to use dsr info to emit syntactic whitespace.

Change summary
--------------
* Serializer can now be run with/without original wikitext source.
  Added --oldtext and --oldtextfile (copied from selser mode) to pass
  original text to the serializer.
* Before serializing, DOM is normalized to eliminate syntactic
  whitespace (with original source) or newlines (without original
  source).
* Separators are emitted separately in serializeDOM -- space before
  current node and first child, space between children, space between
  last child and node.
* List handler fixed up to emit newlines between list items rather than
  include them within the list item.
* Fixed getDOMRange to add empty spans only when needed -- subtle bug.
* Changed DSR computation to associated 1 char with <br/> nodes
  and fixed paragraph wrapper to emit syntactic newlines to keep
  dsr computations unchanged.
* Deleted unneeded normalizeWhitespace dom-post-processor pass.
* Added migrateTrailingNLs dom-post-processor pass to move trailing
  nls out of dom-nodes that end lines in wikitext so that they can be
  emitted as separators during serialization.
* Replaced all outerHTML calls with innerHTML calls in DOMPostProcessor.

Parser tests summary
--------------------
* Added --use_source parser tests mode to run wt2wt tests with/without
  original wikitext passed to the serializer.

* 697 wt2wt tests pass without original source passed in;
  785 wt2wt tests pass when original source is passed in.
  786 used to pass before the refactor.
  (+1 and -2 for a net of -1 since refactoring)
  * one failure because of use of newline as semantic list separator
    fixing this requires inserting a meta-tag possible instead of the
	newline.
  * one failure because of missing dsr[0] value on a mw:Entity which
    leads to loss of a single whitespace char before the entity.
* 2 additional wt2html tests passing.
* 478 html2html tests now passing (instead of 457 on master).

Change-Id: I7dfb1b796021073afd80b6891cdb48469cc2cbde