Major implementation and unittests update.

* Minor bugfixes for last commits (endswith, getItem, ...).
* getItem $default parameter gets default value NULL.
* Don't support PHP4. This is PITA. PHP version >= 5.
* Prefer is_null over '=== NULL'.
* Introduce Stack::diffAssoc method.
* Better name for cloned Stack instances.
* Reorder methods of Sanitizor lexicographically.
* Stack dump $format parameter becomes string
  (consistent with Sanitizor::dump).
* Major improvements for Stack::dump and Sanitizor::dump.
* Major redesign of Sanitizor (hooks, flow diagram).
* Introduce Sanitizor::removeRule.
* Introduce Sanitizor::registerFilter for custom filters.
* Fix documentation. Introduce tutorial in README file.
* Update and extend README file.
Dynamic class names create too many WTFs in older PHP releases.
Introduce documentation for old Sanitizor design.
Add documentation files for tutorial.