Squashed commit of the following:

commit 1e4fab9
Author: John Watson <[email protected]>
Date:   Sat Feb 10 13:52:48 2018 +0000

    Try a local install

commit 89a94cb
Author: John Watson <[email protected]>
Date:   Sat Feb 10 13:39:04 2018 +0000

    text-encoding inst did not work

    let's see if the new test seems to be causing the problem

commit 960d24e
Author: John Watson <[email protected]>
Date:   Sat Feb 10 13:25:50 2018 +0000

    install text-encoding

    Not sure why this dependency is brought in to the tests.  Try to install via .travis.yml using:

       npm install text-encoding

commit b9883e3
Author: John Watson <[email protected]>
Date:   Sat Feb 10 11:43:03 2018 +0000

    Improve documentation of Playable class

commit 405700e
Author: John Watson <[email protected]>
Date:   Sat Feb 10 10:47:43 2018 +0000

    Amend Playable typeclass definition

    The existing definition has toMelody defined as:

       toMelody :: p -> Melody

    which is sufficient for generating Melody from a Midi Reecording.  This is now changing to:

       toMelody :: p -> InstrumentChannels -> Melody

    which is required for other sound sources which may need to generate MIDI note channel numbers by looking up the instrument names (which is the case for PSoM sources).  Sources that don't need this argument may simply pass in empty.