[Enhancement] Adding cache-writer to improve delta performance

This is leveraging broccoli-caching-writer to prevent unwarranted execution of the write function when the sourceTree has not changed.

`less.toCss` is pretty big offender in terms of performance, so CacheWriter will only ever invoke the `updateCache`, previously `write`, function when needed.
It's a convenience so that `var c = LessCompiler(args)` will act as a constructor. Normally you would have to use `var c = new LessCompiler(args)`. [Ref](http://stackoverflow.com/questions/22211755/function-f-if-this-instanceof-f-return-new-f)