Link queries

This PR implements link queries i.e., it extends the query methods to support queries across links. For example:

    RealmResults<Owner> owners = realm.where(Owner.class).equalTo("cat.name", "Max").findAll();

*Notice*: The string part is not working due to how Java/core handles strings. We will fix it ASAP.

@bmunkholm @emanuelez 
The `Array` is passed to a C++ method in the end. Do you know a way to pass an `Iterable` through the JNI easily?