Added support for mongodb $addToSet but named its syntax to $addUnique

Fixed #140
Opted for `$addUnique` because it is much simpler than `$addToSet`.
Below is an example usage:
```
{
  "persons": {
    "$addUnique": ["me", "you"]
  },
  "colors": {
    "$addUnique": "pink"
  }
}
```