Knipse

* `create` subcommand for creating catalogs by reading image file names from stdin
* fix loading image paths containing spaces

* upgrade `twine` in order to have working checks with markdow readme + history

* set `long_description_content_type` to `text/markdown` for proper rendering on [PyPI](https://pypi.python.org/pypi/knipse)

* `check` subcommand for checking existence of files in catalog
* `check` subcommand walks folder structure and checks each catalog found if no catalog is specified
* drop support for Python 3.5
* `Catalog` instances can be serialized to xml
* `Catalog` instances can be iterated and compared for equality

* `Catalog` class for parsing catalog xml files
* `ls` subcommand for listing files in catalog

* fix linter error