test: add Python 3.4 compatibility

* remove 3.2 compatibility claim from setup.py
* add 3.4 environment to tox.ini
* add version-dependent mock imports, becomes unittest.mock from 3.3
forward. Put all the imports in tests.unittest and import locally from
there to centralize version-detection logic.
docs: document run content feature analysis
acpt: add par-insert-paragraph.feature

Acceptance test for Paragraph.insert_paragraph_before() API method.
doc: small refactoring on Document.add_paragraph()

Document.add_paragraph() can rely on Paragraph.add_run() to add the
text.
doc: refactor some names

‘p’ has generally come to be reserved for a <w:p> element and
‘paragraph’ used for a Paragraph instance. Similaryly ‘r’ indicates a
<w:r> element and ‘run’ an instance of Run.
para: add Paragraph.insert_paragraph_before()
docs: add user docs for insert_paragraph_before()
acpt: add run-clear-run.feature

Scenario for Run.clear() API method.
run: add Run.clear()
acpt: add run-add-content.feature

With scenario for Run.add_tab()
run: add Run.add_tab()
acpt: add scenarios for Run.text getter and setter

Both handling text containing embedded \n, \t, and/or \r characters.
run: add Run.text getter

Upgraded to handle more of the run content elements.
run: add Run.text setter
text: support mixed text in existing methods

By using run.text = text instead of run.add_text() in text adding
methods, the parsing logic to interpret newlines and tabs is invoked
automatically.
acpt: add par-clear-paragraph.feature

With scenario for Paragraph.clear()
para: add Paragraph.clear()
para: reposition a couple test fixtures

No code changes.
acpt: add par-set-text.feature

with scenario for Paragraph.text setter
para: add Paragraph.text setter