Add an define method to access stylus' define javascript api.

I needed this method to create variables for custom theming. It's essentially just a simple wrapper that shells out to the define function on the stylus object. Works as a class method on Stylus, e.g:

`Stylus.define "mycolor", "red"`

which uses the `define` api to create a `mycolor` global variable.  Also takes a :literal option in case you need to define CSS keywords.

Also included some passing tests.  I'd like to see this in the next ruby-stylus version in some form or another, so let me know what you think!