Port to Python 3, add combo-device, better testing, etc

[Gadgetoid]

The "combo" device will accept a "config.yml" which details the type, config and location of each plasma strand.

Plasmactl has been ported to Python 3.

Support for Python 2.7 will be dropped as of these changes.

This should fix #4, #3 and #2

[coveralls]

Pull Request Test Coverage Report for Build 939818270

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 14 unchanged lines in 2 files lost coverage.
• Overall coverage increased (+58.5%) to 79.834%

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Files with Coverage Reduction	New Missed Lines	%
plasma/ws281x.py	1	96.3%
plasma/core.py	13	69.09%

Totals
Change from base Build 363562633:	58.5%
Covered Lines:	289
Relevant Lines:	362

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💛 - Coveralls