Implement MIDI clock

Implement MIDI clock

Squashed commit of the following:

commit 68f45a23cef15b544dc1559b72c8e28f7bd4aa3e
Author: Erik Mackdanz <erikmack@gmail.com>
Date:   Fri Sep 22 01:33:50 2017 +0000

    Update README with clock usage

commit 49f20cfae37f0ad70a18809443b88ffe054d0da0
Author: Erik Mackdanz <erikmack@gmail.com>
Date:   Mon Sep 18 23:37:23 2017 +0000

    Support sending MIDI start before next MIDI clock (was concurrent)

commit 82023cb5f6052153084c7b894f20ac99f6a488a5
Author: Erik Mackdanz <erikmack@gmail.com>
Date:   Wed Sep 13 03:09:01 2017 +0000

    Fix segfaults at end-of-song

commit 0d0125c8492d915a11221a061481a432bc8a7b78
Author: Erik Mackdanz <erikmack@gmail.com>
Date:   Wed Sep 13 02:50:26 2017 +0000

    Fixes for clock drift

commit fe891f24ce27026e09c4b03dc3beb9ed5236260d
Author: Erik Mackdanz <erikmack@gmail.com>
Date:   Sat Sep 9 18:21:20 2017 +0000

    Sync first pattern note to next clock beat, remove slow-sync client

commit 0ccbc88d19863f75c603439e20c1fc216000fa6a
Author: Erik Mackdanz <erikmack@gmail.com>
Date:   Sat Sep 9 01:31:41 2017 +0000

    Implement as slow-sync client

commit 47e5f925f50724863bec82a2b3cebd6145a8fa25
Author: Erik Mackdanz <erikmack@gmail.com>
Date:   Fri Sep 8 02:56:37 2017 +0000

    Remove some spurious printf's

commit f9b8f01d4f572ca919a06dfb965565d3a24ef0ed
Author: Erik Mackdanz <erikmack@gmail.com>
Date:   Thu Sep 7 22:17:16 2017 +0000

    On play message, wait to start until next tick

commit de4d8236a47263714b9f9f3cd86fba00ec32c649
Author: Erik Mackdanz <erikmack@gmail.com>
Date:   Thu Sep 7 02:26:22 2017 +0000

    Fix clock tick interval, also ensure midi messages declare their size

commit ca5e9a59eafe6c9d9b4ee99d82082ec9544f0fcd
Author: Erik Mackdanz <erikmack@gmail.com>
Date:   Wed Sep 6 03:42:41 2017 +0000

    Clock emitted before transport starts

    Getting closer, but clock tempo exceeds song tempo for some reason (song tempo may be too slow)

commit 3e74495c4f2dade7b039234b4e82340910f4584f
Author: Erik Mackdanz <erikmack@gmail.com>
Date:   Mon Sep 4 03:48:20 2017 +0000

    Fix a timing shift by filtering end-of-pattern-really-next-pattern events

commit f06a5238ec2894c084748d5ee7287ee560f8e1c1
Author: Erik Mackdanz <erikmack@gmail.com>
Date:   Sun Sep 3 23:40:05 2017 +0000

    Add parsing of clock definition, wrap config port

commit 18bf009c1774a08a683f850c6bc92e079499af78
Author: Erik Mackdanz <erikmack@gmail.com>
Date:   Sun Sep 3 20:52:19 2017 +0000

    Wrap jack port in a type that contains clock numerator/denominator

commit 091369ca14f75e9c32c259206086d1c162b993b8
Author: Erik Mackdanz <erikmack@gmail.com>
Date:   Sun Sep 3 17:46:06 2017 +0000

    Fix for clock signal, it kind of works!

commit 2a60e79b31f4c86dc6bb0499ba6d368cf24ae2ef
Author: Erik Mackdanz <erikmack@gmail.com>
Date:   Sun Sep 3 17:32:51 2017 +0000

    Draft of sending clock signal

    Unverified, hard-coded, seems not to work