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 
The currently implementation of `DateTime` in core are using integers. You cannot specify `DateTime` directly according to @rrrlasse. If you look around the binding's query mechanics, you find the same other places.