Updated dial widget

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.