Improve stack trace printing in html gui
add ability to specify scsynth port
print exception when found in JSON parsing or message dispatch
Make basic_mixer synth a chained in/out bus synth:

* its in-bus will be the job's FX audio bus
* its out-bus will be the studio mixer
Add synth info for basic_mixer and new replace fx synths
Add active support to vendor directory:

* Currently only using Hash with indifferent access
basic-mixer is now basic_mixer
add another useful comment (need more of these)
Move to new Job audio bus infrastructure:

* Each job now has its own audio bus (only allocated if a synth is triggered)
* FX synths are now chained across a single audio bus (now no need for an
  allocation/de-allocation of an audio bus per FX synth)
* Each job now has its own basic_mixer synth (currently only supports amp)
* On job kill, job mixer amp is slid to 0 and then killed - this is to
  stop annoying pops when killing synths mid phase
* Further work on node classes (still needs more thinking)
* Future work is to enable user control of job mixer (fading in and out
  individual job audio)