Basic support for link queries

This PR adds support for link queries - see issue #617. The core library does not support:

* queries on binary data (`NSData`)
* case insensitive queries on strings (`NSString`)
* predicates `BEGINSWITH`, `ENDSWITH`, and `CONTAINS` for strings

@alazier @jpsim @bmunkholm 
We don't want to have to get the table or call ```schemaForTable``` here - it would be much preferred to simply get an existing version of the schema from the Realm (which would need to be passed into this method). If we were able to pass in the current Realm this whole block of code would then be something like:
```objc
if (!firstProp.objectClassName) {
    @throw ...
}
secondType = realm.schema[firstProp.objectClassName][secondColumnName];
```