Linux support

Currently this project only supports OSX. Using the [blueutil](http://www.frederikseiffert.de/blueutil/) utility.
In Linux there's a bluectl command, but it runs interactively. Running an interactive command requires state, something that blueutil does not require. In fact the platform/darwin.py module is really simple.

It'd be possible with some combination of a FSM and `sh.Command(out=callable(line, stdin))`

Start by reading platform/darwin.py to understand the existing platform implementation and how the project invokes blueutil. Investigate the stated bluectl interaction and FSM approach, then verify that Linux support can control the device through bluectl without requiring persistent state.

Written by the indexing model from the issue text.