Merge branch 'release/0.2.1' into develop
refactor oxml to lxml.objectify

* validate lxml custom element classes approach
* implement initial new-style oxml classes
* refactor pptx.presentation to use oxml
* add functions oxml_parse and oxml_fromstring, and perhaps others
  to oxml.py and import those, and not etree in test_presentation and
  perhaps other modules, just to prevent any rogue parsing.
* explore lxml.objectify
* re-refactor pptx.presentation from custom element classes to
  lxml.objectify
* remove custom element class code from oxml
Merge branch 'feature/objectify' into develop
Support reading from and writing to file-like objects.
Refactor tests and docs for open/save stream

* Add acceptance test scenarios for open from and save to stream
* Refactor open/save signatures throughout to use *file* instead of
  *path* as target parameter name
* Refactor docstrings throughout to reflect option of file-like object
  as alternative to path string
* Remove behavior of appending '.pptx' suffix to path if not present
  on save
* Remove trailing whitespace formerly required to get TextMate code
  folding to work properly
* Many other PEP8 fixes
* Remove ``path`` property from FileSystem API
* Factor ``path`` parameter out of ``BaseFileSystem.__init__()``
* Reorganize XxxFileSystem unit tests in test_packaging.py
* Remove trivial legacy tests
* Remove unused methods from testing.py
Prepare v0.2.2 for release
Merge branch 'release/0.2.2'

* Refactor XML handling to lxml objectify
* Add support for open/save presentation from/to file-like object