Added ofxUIEnvelope, ofxUIEnvelopeEditor and associated changes in ofxUICanvas.h and ofxUI.h

Hey Rez!

Here's the src for the envelope editor I was showing you. Hope it's not too messy! I tried commenting in most places, hopefully it all makes sense. Let me know if it doesn't!

![screen shot 2014-01-01 at 5 36 34 pm](https://f.cloud.github.com/assets/4587373/1844338/8747e62e-751c-11e3-8d06-bb22cdbb9694.png)

If anyone else reading this wants to see it working, here's a short example where i'm using it to control amplitude and frequency envelopes in an instrument editor.

https://www.youtube.com/watch?v=BK0f3CicNR4

Cheers,

Mitch