Add GUI and tidy up

[dangarfield]

A lot of tidying up as well as added a dedicated GUI with sliders and event listeners for changes and actions. I'll help @skal65535 with the rest of his work after this. Exciting times!

[hunar4321]

That is awesome! Thanks I'm a bit discouraged now to work on the C++ version when seeing the JavaScript version advancing so swiftly!

[skal65535]

very nice! The UI is very clean and much more polished than my cheap attempt at CSS hacking ;)

Few things i noticed:

• we lost the 'r' and 'o' key to generate new random parameters and reset, respectively. Although we it's still available through the UI, having it as key stroke was useful
• the hash value in the URL seems to be lagging one behind
• changing the hash in the menu doesn't trigger a corresponding new set of parameters to be drawn
• i would be nice to have a little color dot or icon to remind the actual color as visual help (when trying to play with pairs of particles for instance, it's easier to find the corresponding pair of params on the UI if you have a visual color help imho)

For the particles, what's missing is a per-particle radius (instead of the global cutOff param) which produced interesting pattern, as well as the main loop fix that updates the velocity separately from position. I can have a look at that later, leaving the UI to Dan's expertise...