open/save round-trip, 122 unit test cases
removed doc/_build from tracking
broad additions to working documentation
refinements to default template
backtrack on table-driven model part constructors idea
add InvalidPackageError exception
major refactoring of pptx.packaging module
major development thrust on pptx.presentation module
incrementally adding symbolic constants for content types, e.g. CT_SLIDE
added Partname class to encapsulate partname parsing logic
unit test coverage brought to 98%
add scrap code for placeholders
quick fix on _Relationship method name refactoring
Update README.rst with status in prep for push