Add a scala_test rule to Bazel

This implements a scala_test rule analogous to the java_test rule. It uses the standard scalatest library, and adds the corresponding jar(s) to the WORKSPACE file. The rule is tested by adding a proof-of-concept test target to the scala examples.

Link to discussion thread: https://groups.google.com/d/msg/bazel-dev/5Kbbwr11XOA/znTY-H1DBgAJ

Fixes #503.

--
Reviewed-on: https://github.com/bazelbuild/bazel/pull/657
MOS_MIGRATED_REVID=113526680

WORKSPACE

diff --git a/WORKSPACE b/WORKSPACE
index ebe5700..1cb41a8 100644
--- a/WORKSPACE
+++ b/WORKSPACE
@@ -52,3 +52,11 @@
     url = "http://downloads.typesafe.com/scala/2.11.7/scala-2.11.7.tgz",
     build_file = "tools/build_defs/scala/scala.BUILD",
 )
+
+# only used for the scala test rule
+http_file(
+    name = "scalatest",
+    url = "https://oss.sonatype.org/content/groups/public/org/scalatest/scalatest_2.11/2.2.6/scalatest_2.11-2.2.6.jar",
+    sha256 = "f198967436a5e7a69cfd182902adcfbcb9f2e41b349e1a5c8881a2407f615962",
+)
+