Implement Array operators

[olliemath]

Added has, hasAny etc. for arrays - with some docs and tests

[ishirav]

Hi Oliver,

Thank you very much for your high-quality pull request. I see that you've included tests and documentation, which is great.

I'm currently working on finishing up v2.0 of the ORM, which will include support for these array functions (and many more), using a much richer and natural syntax for using database function. For example you could do something like this:

# Find people with more than two cars
Person.objects_in(db).filter(F.length(Person.cars) > 2)

In view of this, I prefer not to add new functions in the old format (although v2.0 will still support them).

Please don't see this as any kind of disapproval of your work. I hope this won't discourage you from contributing additional pull requests in the future.