Improve auto-paragraph to preserve newlines and handle edge-cases better.

This is a very large commit that includes numerous improvements to the
AutoParagraph injector.  These are:

* Rewritten flow control of the injector to use almost exclusively
  binary conditionals.
* Improved inline documentation with "State" comments, which give concise
  examples of what the token stack looks like at flow points.
* Documentation for all flow branches, even those with no actions.
* Factoring out of common operations to improve readability, especially the
  new iterator private methods.
* Expanded test-suite which covers new flow points, and corrects some errors
  in previous cases.

Signed-off-by: Edward Z. Yang <edwardzyang@thewritingpot.com>

NEWS

@@ -26,6 +26,10 @@ NEWS ( CHANGELOG and HISTORY )                                     HTMLPurifier
   'phpt', 'vtest', etc. in order to only execute those tests. This supercedes
   the --only-phpt parameter, although for backwards-compatibility the flag
   will still work. 
+! AutoParagraph auto-formatter will now preserve double-newlines upon output.
+  Users who are not performing inbound filtering, this may seem a little
+  useless, but as a bonus, the test suite and handling of edge cases is also
+  improved.
 - Fix two bugs in %URI.MakeAbsolute; one involving empty paths in base URLs,
   the other involving an undefined $is_folder error.
 - Throw error when %Core.Encoding is set to a spurious value. Previously,