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 
`String.split()` is a strange beast, and quite unpredictable.
Guava's `Joiner` is **way** better. Maybe we can consider including just that class from it. And `Splitter`