Add support for pixel iso angle

[FabrizioC]

First of all: Thanks, great job!

According to this schema: http://www.sastgroup.com/images/iso-angles.gif adding a parameter in Isomer initialization the "real" iso angle or "best for pixel representation" iso angle could be selected, e.g.:

if ( options.isotype == 'pixelperfect' )
  this.angle = Math.atan(0.5);
else
  this.angle = Math.pi/6;

... or something like this.

[grant]

👍 That would be sweet.