Add support for Transform

@imlucas 
Two new API methods:
* `createGeneratorStream({size: n})` - a Transform stream to generate documents (in Generator.js)
* `createPopulatorStream({uri: "mongodb://...", collection: "test"})`. - a Writable stream to insert docs to mongodb (in Populator.js)

I kept the old codes since I am not very confident to refactor everything for now. I will do that in a future pull request. Thanks!