volume slider, timbre presets, tweaks

- volume slider
- synth presets dropdown menu
- synth presets come from lib/Tone.Preset.js
- synth goes through reverb
- added some CSS colors that we can totally change
- reorganized:
  + took all javascript out of the index.html, moved it mostly to a new file called main.js
  + main.js calls setup() to initialize everything when the page loads, and draw() for the animation loop. Also listens for events like key/mouse press.
  + sounds.js --> stuff that has to do with sounds (i.e. Tone and note/scale mappings)
  + ui.js --> user interface elements created by javascript. For now it's just the dropdown menus.