Refactor printcore.py

[rockstorm101]

Split printcore.py into two. All the functionality related to directly communicating with serial ports or TCP sockets is moved into device.py. This way this logic is abstracted from printcore.py, which is now independent of the underlying connection type.

This reduces the complexity of printcore.py and increases code modularity. Plus it has the advantage of making it simple to add more connection types in the future (if any).

Notes:

• This refactored code passes all tests on test_printcore.py (with a very slight adjustment), so (in theory) merging this PR this does not change/break current API.
• I've tested it on my printer through a serial connection and works fine so far, however, I could not test the TCP connection. The goal has been to keep the code logic the same so expectation is no regression issues occur.
• I've named it device.py and Device as the main class but I'm open to suggestions since I'm not entirely happy with the naming.

As always, any comments/suggestions are appreciated and I'll be happy to address them.

[volconst]

• I could not test the TCP connection.

Try to connect to https://github.com/kliment/Printrun/blob/master/testtools/mock-printer.py

[rockstorm101]

Try to connect to https://github.com/kliment/Printrun/blob/master/testtools/mock-printer.py

Of course, thanks! I forgot about that. It communicates without issues with that mock-up on my machine, yey! \o/