Add children attribute feature + Groovy test case (RiverMongoChildrenTes...

Hi Richard, I've implemented a new feature in the river to allow index an array of a mongodb document. Each element of the array will be indexed as a single document in elasticseach. 

E.g.: At my work we have a purchase order composed by many items:

Order.json
```
    {
    	...
    	"items":[
    	   { "name": "Item #1", ... },
    	   { "name": "Item #2", ... },
    	   { "name": "Item #3", ... }
    	]
    }
```

Due some business requirements we need and index of all order items. This now is possible creating a river with the children attribute.
Have a look to the RiverMongoChildrenTest to see more details and spec. I wrote the test cases using Groovy and GMongo (way cleaner and less code than using Java).