Update README.md

Bulk API documentation referenced by URL
Merge pull request #32 from llonchj/master

Update README.md
Rewriting all tests using JUnit
Right now we have only tests running as Main() programs.
It could be nice to have tests running while releasing the project.
Note that you should have rabbitMQ server running locally if you want to run tests.
If not, you will get a `WARN` message when launching tests.

Closes #31.
Using multiple threads to consume rabbitMQ messages.

```sh
curl -XPUT 'localhost:9200/_river/my_river/_meta' -d '{
    "type" : "rabbitmq",
    "rabbitmq" : {
        "num_prefetch" : 0,
        "num_consumers" : 1,
    },
    "index" : {
        "flush_interval" : "5s",
    }
}'
```
We also removed `ordered` and `bulk_timeout` options as we don't need
them anymore.
Closes #11.