partest test interface for sbt

dummy implementation that simply triggers from sbt and prints what's happening

To trigger the partest testing interface, add something like this to your
`build.sbt`:

```
// for testing with partest:
libraryDependencies += "com.typesafe" % "scala-partest-interface_2.10" % "0.1" % "test"

testFrameworks += new TestFramework("com.typesafe.testing.partest.Framework")

definedTests in Test += (
  new sbt.TestDefinition(
    "partest",
    // marker fingerprint since there are no test classes
    // to be discovered by sbt:
    new sbt.testing.AnnotatedFingerprint {
      def isModule = true
      def annotationName = "partest"
    }, true, Array())
  )
```

SBT's `test` task will now see a test with the fingerprint that matches
the fingerprint in the array of fingerprints (tests, really) returned by
`com.typesafe.testing.partest.Framework::fingerprints`.

SBT unfortunately hardcodes fingerprint matching, so I don't see
how else to shoehorn partest into this approach.
reflective invocation of partest's sbt runner

reflectively instantiate `scala.tools.partest.nest.SBTRunner`,
passing test classloader from sbt to partest's sbtrunner

neat trick: cast to the structural type `SBTRunner`,
and let scalac synthesize the reflective call for
invoking `execute` on the runner