Added support for $elementMatch
Added support for $elementMatch

Fixes issue 76.

Permits creation of more complicated queries like:
```
[
    "documents": [ 
        "$elemMatch": [ 
            "value": "1234" 
        ] 
    ] 
]
```
PlanTeam/MongoKitten,OpenKitten/MongoKitten,PlanTeam/MongoKitten,OpenKitten/MongoKitten