Release string interpolation in @import statements.

[anru]

Warning! Many api changes!

[anru]

api changes:
Parser.parse(str, callback(err, tree)) => tree = Parser.parse(str); // throws err;
css = tree.toCSS(options, vars) => tree.toCSS(options, vars).then(callback(css), errback(error));

And root's tree.Ruleset.eval returns promise object instead of evaled ruleset,
tree.Import.eval returns promise object too.

Also included when.js library for deferred functionality.

[arnaudgaudin]

Excellent ! Thank you very much, can't wait to test this new functionnality in my project ! Just hoping that leafo will add this to lessphp...

[cloudhead]

what's with the API changes? If you want this merged, you'll have to open another pull request with only the relevant changes.

[anru]

Without API changes ? it is hardly to possible, becouse toCSS() async operation if @import has string-interpolation (in contrast with Parser.parse)
Or without when.js library ? It makes sense, for ex. rewrite to use events.EventEmitter instead.

[paulyoung]

Not sure about the api changes, but a much needed feature.