README overhaul and sewing the seeds for OR queries.

Basically, I've:

* Re-written the README to make it more user friendly (and have merged in the recent changes from Luke and Stu).
* Split out the TODO list into a separate TODO.md, as that's only relevant to contributors.
* Added notes in the code about how we could do OR queries and added some more (currently failing) test coverage to pave the way for them.