Implement TLS version of fixture

[s-weigand]

This PR implements a second fixture ftpserver_TLS, which is a TLS encryped version of the fixture.
Other changes are:

• introduced the possibility to configure the scope of the fixtures
• renamed BaseMPFTPServer to FunctionalityWrapper
• refacored the structure of the code
• updated setup.py to reflect current state of the plugin and hopefully render properly on PyPi
• added more discriptions to HISTORY.rst
• extended and improved documentation

[coveralls]

Pull Request Test Coverage Report for Build 82

• 294 of 294 (100.0%) changed or added relevant lines in 3 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 100.0%

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Totals
Change from base Build 78:	0.0%
Covered Lines:	356
Relevant Lines:	356

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💛 - Coveralls

[oz123]

@s-weigand thank you for this huge effort. I am sorry it took me a while to review this.