Source node from string with map

This pull request is based on #46 and #47.

It added a method [SourceNode.fromStringWithSourceMap](https://github.com/sokra/source-map/commit/5bc8cce362d5b331816c80aaabb6ba15a76c499e).

It converts a SourceMap into a SourceNode. It's useful if embedding other files that already bring a SourceMap.

This is an optional step 3 for composing SourceMaps.