Allows the core of .add_paragraph() to be tested integrated with oxml
while using mocks for the remainder.

* place api-add-heading.feature back into WIP until Paragraph.style
reimplementation is completed

to use new style API

to use new style API

* organize feature analysis for styles into a hierarchy

* Introduce a hierarchy for text analysis

The tests/ directory was only partially included in the source
distribution uploaded to PyPI, such that the test suite could not be run
from that distribution. Modify MANIFEST.in to include all .py files in
the tests/ directory.

* add specific Given for a document having the built-in styles
* constrain 'Given a blank document' to not have the built-in styles
* revise some steps to make them more concise
* extract bool_vals and tri_state_vals to helpers

Add steps to check column and cell width are properly applied to a new
table.

* Add width parameter to BlockItemContainer.add_table()
* Temporarily stub down-call-stack methods and tests with width=None
  until they are rewritten in next commits
* add Document._block_width to calculate width of table (page width less
  margins).

* Fix Describe_Cell.it_can_add_a_table() to work until it can be
rewritten in next few commits.

Add width parameter.