Gradient

This pull request accompanies #41 in -svg and #136 in -lib
*** Please do not merge yet ***

SVG and Cairo have built in support for different types of radial gradients. 
SVG allows the specification of a focal point in addition to a center and radius from which the gradient eminates.
Cairo allows the specification of a torus on which the gradient lives.

The easiest way to understand the difference is to look at: http://www.w3.org/TR/SVG/pservers.html#RadialGradients

 and

cairo_pattern_create_radial () in: http://cairographics.org/manual/cairo-cairo-pattern-t.html