added benchmark script and documentation

Exciting benchmark utility detailed in `benchmark.md`! Also added documentation showing how to stream directly without using a dump file. #21
^ So first off I love the benchmark script and I think IT'S AMAZING.

But I question putting the script in this repo. It's supposed to be a base library to be used by other utilities (e.g. plugins and CLIs). If you want people to use the script, wouldn't the easiest way be to create a separate module that defines a `bin` so that people can

```
npm install pouchdb-replication-stream-bench
pouchdb-replication-stream-bench --db [http://localhost:5984/stream_benchmark] --records [500] --batch [100]
```

?

Or if you disagree, then at least some more instructions here explaining where to find `benchmark.js`; some people might take awhile to find it. :)