Update dial_render.js

Fixes issues with dial widget, reported here:
http://openenergymonitor.org/emon/node/6222
and demonstrated here:
http://emoncms.org/Tricx/demo

Changes
- maxvalue is always the end limit of the range.
- The „minvalue" or starting point is calculated based on the maxvalue and dial type by using center position/angleoffset, automatically.
- Added a new dial type with blue to red color scheme and center at north west ot meet outside temperanges in degrees.

Best
Tricx
Merge pull request #258 from Trickx/patch-1

Update dial_render.js