Added source maps support for concatenated assets

I've used [convert-source-map](https://github.com/thlorenz/convert-source-map) and [combine-source-map](https://github.com/thlorenz/combine-source-map) libraries which are also used by browserify.
See [this blog post](http://thlorenz.com/blog/browserify-sourcemaps) on why they've chosen to include the sources (sourcesContent) in the map and keep the map itself as a comment.

The BundledAsset's constructor doesn't seem to be the best place for such strictly source-map-related code, so if you have any suggestions, I'll be glad to fix it.