Add better support for conditional styles and code reuse

[adamldavis]

In comparable systems, you can define a method which is called elsewhere, pass parameters, and conditionally add styles. GrooCSS should easily support this with a few additions.

For example, something like:

def mycolor(alpha) { styles {
   if (alpha == 0) color '#123'
   else color rgba(0,0,0,alpha)
}}

and elsewhere within a style-group:
add mycolor(0.5)

[adamldavis]

This has been added. I've added the following within a test:

        def mycolor = { alpha -> styles {
            if (alpha == 0) color '#123'
            else color rgba(0, 0, 0, alpha)
        }}
        assert mycolor(0).styleList[0] == new Style('color', '#123')
        assert mycolor(0.5).styleList[0]==new Style('color', 'rgba(0, 0, 0, 0.50)')

You can't create inner methods within a closure, so I've used a closure instead.