Update to Spire 0.9.0.

Updating involves a number of things:

 1. Bump Spire version to 0.9.0.
 2. Bump Scala version to 2.11.4.
 3. Bump ScalaTest/ScalaCheck to most recent versions.
 4. Implement Torsor[DateTime, Duration].
 5. Replace use of Double with Rational.
 6. Add scalacOptions to build definition
 7. Add tests (using Spire's laws)

We need a better README, some decent documentation, and
more tests before anyone even thinks about a release,
but this package is starting to come together a bit.