Added OR and NOT operators for ActiveRecord::Relation instances

[bcardarella]

Now we can do:

Developer.where(:salary.gt => 70000) - Developer.where(:company_id.not_eq => 2)

[bcardarella]

On second thought, maybe these operators are best reserved for UNION, INTERSECT and EXCEPT. It seems that UNION has already made it into Arel/master: brynary/arel@d532b7e

[bcardarella]

...which I've added here: rails/arel#24

So if it gets accepted all three SQL Set Operators will be in there.

What I propose:

+: for merge

&: for intersect

|: for union

-: for except

This is using the convention for the Array class.