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.
It seems like it is a bummer to have non-test dependencies, when one of our goals was to make this project dependency-free.

We've been burned again and again with diamond dependency pains, and for a project that has aspirations to be at the bottom of the stack, a dependency on a particular version of scalacheck or discipline may cause pain down the line.