Add support for composite keys

[libjack]

I've pushed a change on my fork to support composite keys, and works fine in my use. However, as it stands, it won't work if a CF has both a composite key as well as composite column (unless composites are identical). This is because ToClojure/AbstractComposite only looks for the :c-serializer option.
Before I make any of those changes, wanted to discuss with you (and perhaps mstump) about necessary fix. Also, since a CF could have Composite values as well, might want to provision for that.