Add support for specifying comparators in set ops

[rcoh]

This commit adds support for specifying comparators in union,
difference, and intersection (#1339). The specification of comparators allows
for library users to specify _.isEqual for deep equality or to
specify their own methods that match a subset of paramters. It
also adds deepUnion, deepIntersection and deepDifference functions,
convienience functions that specify _.isEqual.

Under the hood, it actually allows the specification of a "contains"
function into several relevant relevant functions. This is created from
the comparator with a closure. The code to allow this a little cumbersome.
I'm open to suggestions on better methods of passing through a containment-
checking function -- one possibility would be to store the containment
function of the moment on this.

Let me know your temperature for merging these changes.

[jbrains]

+1

I'm curious: why compare stringified objects tests, rather than simply compare the resulting objects?

[rcoh]

Following the pattern of existing tests. I assume it's to avoid relying on the correctness of deep equals

[jbrains]

I see. It seems risky to do it that way, but I understand the desire to write the assertions concisely. I'd probably write them out "the long way", myself. :)

Thanks again.

[jashkenas]

Very lovely stuff — but probably just slightly too esoteric to ship in core Underscore. I'd recommend moving this patch over to Underscore-contrib.