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