Teach with_merged_synth_defaults and with_synth_defaults to accept
a single arg hash as well as as vararg pairs.
Remove pad functionality. The focus for Sonic Pi is currently for
auto-freeing enveloped synths only.
Minor refactoring
Remove deprecated function comms_eval

(I can't even remember why it was there!)
Remove duplicated synth triggering functionality.
Create and use new node abstraction ChordGroup:

* When a chord is played, all nodes are placed in a new ChordGroup
* ChordGroup inherits from Group, so may be used to control all sub nodes
* Subnodes of a ChordGroup are available with the `nodes` method
* Move resolve_opts_hash_or_array into Util module
Improve to_s of Chord and Scale
defer to play_chord if an Array is passed as the note arg to play
add to_s for Buffer
Return scale as an Array rather than a Scale object:

* Keeps the number of abstractions people need to learn to a minimum