fix example/finitefield.scala: add gcd method
rework project structure

* core sources of spire now live in their own subdirectory
* tweaking the build file, update SBT to 0.12.2
* add new artifact: spire-scalacheck-binding which exports the law checking
  mechanisms

Everything should still work out of the box like it used to, except that a new
artifact is being generated.

$ ./sbt compile # compiles core, scalacheck-binding and examples
$ ./sbt test    # tests core and scalacheck-binding
$ ./sbt publish # publishes core and scalacheck-binding as
                # 'spire' and 'spire-scalacheck-binding'
update ScalaTest to 1.9.1 and fix resulting deprecation warnings