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.
Would we have a Hash trait in algebra? Would it be linked to an Eq? Without it, I don't see a law for it.
```
scala
trait Hash[T] extends Eq[T] {
  /**
   * if eqv(a, b) then hash(a) == hash(b)
   */
  def hash(t: T): Int
}
```