Refs #77. Fixed the failing test case by adding the main document part after it is created
Refs #77. Abstracted the load_relationships logic to work with parts and packages
Refs #77. Updated a test case
Refs #77. Updated a test case
Refs #77. Added StyleDefinitionsPart and ChildPartLoader abstraction
Refs #77. Added failing test cases
Refs #77. Use get_part on the open_xml_package
Refs #77. Added a failing doctest
Refs #77. Replaced zip_path_join with posixpath.join, it does it better, plus it's standard.
Refs #77. Removed this doctest since it's failing and we're not using zip_path_join anymore