Add subprojects; first pass at adding laws and tests.

This commit creates three sub-projects:

 - core: builds algebra.jar; contains type classes (and a few types)
 - std:  builds algebra-std.jar; contains sample instances
 - laws: builds algebra-laws.jar; contains laws and conformance tests

It also ports over the basic laws from Spire for:

 - signed
 - eq/order
 - groups
 - rings

Finally, it adds a very simple standard algebra instance for Int, and
runs the relevant laws tests on it.
I think if I make it a case class then I can't override apply to do the checking I wanted. Unapply is fine but I didn't end up needing it for this example. I'm happy to make it a case class if you think the class will see actual use (I was just trying to get a "minimal implementation" done).