Composed Primary Keys

[lordoffreaks]

Hello

I do some work at https://github.com/lordoffreaks/cactus/compare/develop for getting support for composed primary keys

I prefered to share that here before making a pull request

Greets

[daveWid]

I don't have much experience with composed primary keys, but as long as it doesn't break backwards compatibility (passes all the current tests) then I'm cool with it.

Can you please use tabs and put the braces on their own lines though?