Adds utility methods for getting and appending to content fields

Adds getters for the Hydra serializable types (`long`, `double`, `String`, `List` and `Map`, see `api/readme.md`).
Adds method for appending values to content fields without type-checking, since doing that kind of check reliably is difficult at runtime (there will always be cases where the type is unknown). Additionally, generic types of lists and maps are erased on serialization, so all documents received in a stage will have maps and lists with generic type `Object`.

This resolves #209

There might be more utility methods that could be useful; a quick check through the basic stages show a frequent pattern of `if string, do this and output a string; if list, do something else and output a list`. These methods don't really help with that.