add a scala_check target that only uses scalacheck.

[johnynek]

This could be useful for people that already have scalacheck tests and don't want to convert to scalatest to use them. Also, the dependencies of scalacheck are very thin.

[johnynek]

cc @non we should add some nice assertion macros to scalacheck for this.

[johnynek]

note this:

https://gist.github.com/johnynek/48ab7f060dda1bf0edc7be8229576048

Shows in < 400 lines how to use classgraph + sbt.testing (both java only dependencies which are quite small, each with no other dependencies) to run any test.

cc @ittaiz @andyscott