Nbstripout

* Strip `ExecuteTime` metadata (34, 39, jdriordan)
* Fix Python 3.5 bug: open attributes file only once (40)
* Do not add blank line at beginning of attribute file
* Strip widget state from notebook metadata (42)

* Drop support for Python 2.6, 3.2.
* Add pip install instructions to README (32, oogali).
* Write trailing newline to attributes file (36).
* Uninstall only removes ipynb filter (37).

* If you set either the `"init_cell": true` or `"keep_output": true` in the cell metadata, then these cells will not be stripped out. The former works in conjunction with the `init_cell` nbextension (17, mforbes).
* Fix encoding for Python 2 + 3 (11).
* Add `--is-installed` and `--status` options (29).
* Normalise cell output style (scroll / collapsed, 30, kdmurray91).
* Add screencast (31).

* Use pytest-cram (22, mforbes).
* Add further shields to README.
* Use argparse for argument parsing.
* Add `--attributes` option to specify attributes file (25).

* Python 3 compatibility (16, 21, boeddeker).
* Windows compatibility (18, tt293).
* Add support for AppVeyor (24).

* Add `__version__` and `version` command (12).
* Add bumpversion config.
* Add contributing guidelines (13).