refactor oxml to lxml.objectify

* validate lxml custom element classes approach
* implement initial new-style oxml classes
* refactor pptx.presentation to use oxml
* add functions oxml_parse and oxml_fromstring, and perhaps others
  to oxml.py and import those, and not etree in test_presentation and
  perhaps other modules, just to prevent any rogue parsing.
* explore lxml.objectify
* re-refactor pptx.presentation from custom element classes to
  lxml.objectify
* remove custom element class code from oxml
Merge branch 'feature/objectify' into develop