Add adjoin trick to add identity to pseduo-rings

[johnynek]

http://en.wikipedia.org/wiki/Pseudo-ring#Adjoining_an_identity_element

What we want is something like:

// in Ring object:
  def adjoinOne[T](implicit pseudoring: Ring[T]): Ring[(BigInt, T)] = new Ring[(BigInt,T)] {
    lazy val one = (BigInt(1), pseudoring.zero)
    lazy val zero = (BigInt(0), pseudoring.zero)
   // and follow the rules above.
  }

This would allow us to use a Map as a true ring, not a pseudo-ring, which could be useful in Matrix.scala (but honestly, this is mostly a correctness issue, I don't think matrix.scala often assumes ring.one exists).