Gradients can now be authored, converted, interpreted, and rendered

Included conversion pathways:
- Plain English* :smile: -> Gradient 
- MML gradient -> Gradient
- Gradient -> CSS3 gradient
- Gradient -> SVG gradient

*Effortlessly author gradients with the included gradient parser! No prior knowledge needed.

Note: The upper-right ellipse of gradients.mml renders incorrectly in the _Java_ MW. Surprising, right?