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 
I could write a simple function to split a string. We really don't need a general string splitter: we will always split on `.`.