Rename internal classes with leading underscore
Refactor oxml.py into custom element classes

* Factor pic element out of shapes.py
* Add image scaling to _Image
* Refactor image pixel size into _Image._size
* Move _add_image() from _ShapeCollection to _BaseSlide
* Move XML emission testing to test_oxml.py
* Bring pptx/spec.py up to PEP8
* Contruct _Table only if graphicFrame has table
* Fix _BasePart.add_relationship reuses matching rel
* Fix doc bullet slide example
Baseline chart part and shape documentation

* Reformat spec.py to PEP 8
* Create symbolic names for all remaining relationship types and
content types (except image types)
* Add CT_CHART, RT_CHART, CT_WORKSHEET, and RT_PACKAGE
* Add chart experiment page
* Add CT_GraphicalObjectFrame schema page
* Add CT_Table schema page
* Add CT_Chart schema page
* Add chart object page
* Add chart behavior experiment page
Merge branch 'feature/chart-basics' into develop

Conflicts:
	doc/developer/analysis/schema/ct_graphicalobjectframe.rst
	doc/developer/analysis/schema/ct_table.rst
	doc/developer/analysis/schema/index.rst
	pptx/presentation.py
	pptx/spec.py
	test/test_presentation.py