Created by: jingman
Adds support (via arity) for an extra argument (@backgroundColor) to each of the #gradient mixins.
Being able to declare a background color is especially useful when using translucent color values in your gradient - as the background color will show through.
I am using the relatively new (v1.3) LESS feature for variadic argument support (https://gist.github.com/1933613). This same functionality can be achieved without it, but that solution is a bit less elegant, using backtick evals and interpolation.