Using java annotations instead of scala type aliases does not work

Using java annotations directly has no effect.
One have to use the scala type aliases.

Ex:
```
import io.sphere.mongo.generic.annotations.MongoKey
case class Student(
  @MongoKey("_id") id: String
)
```
does not work.

The following works
```
import io.sphere.mongo.generic.MongoKey
case class Student(
  @MongoKey("_id") id: String
)
```
