Remove old canvas, clean up watchers on reinitialization, better max value support

Hi,

I have improved some bits and pieces:

* Max value is now updated through refresh, as justgage permits that https://github.com/toorshia/justgage/blob/b5398633c52c2027607d93f88432ce176ddac9c6/justgage.js#L778-L779
* On reinitialization old canvas object is removed
* Old watchers are cleaned up when new JustGage object is created.

Let me know what you think.

Cheers,
Igor.