Added multiple shade statements in one line, and nesting

This pull request adds two useful features:

1) The ability to have multiple shade functions on one line. For example

    background: linear-gradient(top, shade(#00F, 20), shade(#00F, -20))

2) The ability to nest shade statements

   background-color: shade(shade(#FA5, -20), 10);

   This is useful if you have variables that derive color from each other. For example `$colors.darkBlue` might have shade applied to derive `$colors.darkerBlue`.