Fixes to finding builder-corrected tags algo.

* The old code relied on the assumption that mw:StartTag
  meta-tags were always the first child of the node they
  marked.  But, this assumption is not always true which can
  be verified by dumping dom post-builder on this example:

  echo "# 
 a\n# b 
" | node parse.js --dump dom:post-builder

  Look at the second li -- there is an s-node that shows up
  between the li and its start meta

* The patch does not rely on this assumption anymore plus fixes
  some unrelated bugs.

* Fixes RTing of the above snippet which is a reduced test case from:
  Wikipedia:WikiProject_Films/Assessment/Tag_&_Assess_2009-2010/009

  This page (and possibly other pages) now RT correctly.

* No change in parser tests in any mode.
  TODO: add new tests.

Change-Id: I5f1a82507d98c0ef3487587a773f4f79f6bd5a46