Source map gen from map

This pull request is based on #46 and #47.

It added a static method [SourceMapGenerator.fromSourceMap](https://github.com/sokra/source-map/commit/347944f2e96db8a893660ed35e9d519d2b2c1203).

It allows to convert a SourceMapConsumer to a SourceMapGenerator.

This is step 1 for composing SourceMaps.