Refactoring for modulable wrappers / Python2 support + SystemTags wrapper

[luffah]

Hello, sorry for the massive PR.

I refactored the code in order to dynamically add features on imported wrappers.
It shall ease creation of new wrappers outside this lib (custom apps) .

Too, i added what i needed:

• python2 compatibility
• wrapper for SystemTags

Lib API notable changes :

• user_id removed from WebDav arguments (using main client user_id)

I polluted again the PR with some lints, documentation and moved confusing files.

Nextcloud.py -> __init__.py
nextcloud/Dockerfile  -> tests/Dockerfile

( Missing test for systemtags, so sorry )
Edit: related issues #67, #66, #68, #5
Changes include

• addition of an option to set alternate auth method and usage as session (working with instruction with NextCloud as nxc: ; too login success is checked with exception)
• addition of AUTHORS.rst
• update requirements versions to allow to use plugin with pytest
• moving descriptive content setup.py to setup.cfg; and add a shortcut to publish in PyPi (one day)
• test.sh script

[luffah]

( @EnterpriseyIntranet Thank you for the free code inspection )

[scrutinizer-notifier]

The inspection completed: 120 updated code elements

[luffah]

@EnterpriseyIntranet @matejak @danil-topchiy
FYI if the PR please you a little : i forked the project.

• https://github.com/luffah/nextcloud-API/tree/main -> stable dev
• https://github.com/luffah/nextcloud-API/tree/develop -> latest changes