Clarify deleting documents in multi-type example

Closes #55.
Use `script_type` instead of `scriptType`

We want to use a standard way for naming river parameters. We are currently using came casing only for `scriptType`.

So we first add the support for `script_type` and deprecate the `scriptType` parameter.

`scriptType` will be removed in an upcoming version.

Closes #62.
Default script engine should be mvel

We don't need to force users to embed the `js` plugin when dealing with scripts.

`mvel` is the current default scripting engine in elasticsearch 1.2.x

For compatibility reason, users still can use the [javascript plugin](https://github.com/elasticsearch/elasticsearch-lang-javascript) but in that case they must set `script_type` to `js`.

Closes #64.