adds new spec/mugl/temp-precip.xml test graph
fixes broken pan min/max treatment; changes pan min/max default from `undefined` to `null`
adds default icon to legend. fixes #52
adds a method to the legend which determines if it should be drawn

adds specs

changes the 'visible' attr to allow for null, and updates its default. Adds a
method 'determineVisibility' which returns the value of the 'visible' attr if
it was set to a Boolean or returns a Boolean based on the number of plots if
the 'visible' attr was set to null, ie if it was not set in the mugl.
moves some code from legend.initializeGeometry to normalizer. adds specs
runs ant graphicstests
removes legend opacity from all instances of renderLegendIcon. fixes #54
adds utility function for parsing booleans
runs ant graphicstests
changes parsing of boolean types to use the new utility function

fixes #53