Refactor testserver into multiple files

[stlehmann]

This PR basically rebases #237 to master and fixes issues with coveralls.

[coveralls]

Pull Request Test Coverage Report for Build 836422268

• 376 of 414 (90.82%) changed or added relevant lines in 5 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-1.0%) to 93.69%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
pyads/testserver/handler.py	18	19	94.74%
pyads/testserver/basic_handler.py	80	87	91.95%
pyads/testserver/advanced_handler.py	164	176	93.18%
pyads/testserver/testserver.py	110	128	85.94%

Totals
Change from base Build 775279533:	-1.0%
Covered Lines:	1559
Relevant Lines:	1664

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💛 - Coveralls

[stlehmann]

Thanks @RobertoRoos for this PR. I made some small changes regarding the whereabouts of the tests. I placed your additional tests for the AdvancedHandler in test_connection_class.py which is where also the tests for the BasicHandler live. I also removed the # pragma statements which eventually results in a lower coverage. But I don't mind that as long as we know where it comes from. Also we can improve coverage by time 😉