implement ShapeCollection.add_textbox()

* add acceptance test 06-add-textbox.feature
* add pptx.api module with Presentation class
* add pptx.oxml module with classes to encapsulate XML access
* add test/test_oxml.py module
* refactor .add_slide to .add_slide(path, left, top, width=None,
  height=None)
* replace ShapeCollection.__sp() with oxml equivalent
* new authoring/editing pass on documentation
* move doc schema/ directory up one level
* add lab/ directory for experimental code
* first-pass code to generate oxml classes