Add a scala specs2 example (pantsbuild#4516)

Adding a specs2 example, so it is more clear how it can be used. Also tested in an IntelliJ project, the tests run as expected as well in both IntelliJ JUnit runner and specs2 runner.

3rdparty/jvm/org/specs2/BUILD

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+# Copyright 2017 Pants project contributors (see CONTRIBUTORS.md).
+# Licensed under the Apache License, Version 2.0 (see LICENSE).
+
+jar_library(
+  name='specs2-junit_2.11',
+  jars=[
+    jar(org='org.specs2', name='specs2-junit_2.11', rev='3.8.9'),
+  ],
+)

examples/tests/scala/org/pantsbuild/example/specs2/BUILD

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+# Copyright 2017 Pants project contributors (see CONTRIBUTORS.md).
+# Licensed under the Apache License, Version 2.0 (see LICENSE).
+
+junit_tests(
+  dependencies=[
+    '3rdparty/jvm/org/specs2:specs2-junit_2.11',
+  ],
+)

examples/tests/scala/org/pantsbuild/example/specs2/HelloWorldSpec.scala

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+// Copyright 2017 Pants project contributors (see CONTRIBUTORS.md).
+// Licensed under the Apache License, Version 2.0 (see LICENSE).
+
+package org.pantsbuild.example.specs2
+
+import org.specs2.mutable.SpecificationWithJUnit
+
+class HelloWorldSpec extends SpecificationWithJUnit {
+
+  "add three numbers" in {
+    1 + 1 + 1 mustEqual 3
+  }
+
+  "add 2 numbers" in {
+    1 + 1 mustEqual 2
+  }
+}

examples/tests/scala/org/pantsbuild/example/specs2/README.md

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+# Scala Specs2 Example
+
+Pants supports Scala Specs2. In order for it to work you have to extend the `SpecificationWithJUnit` class, because Pants uses JUnit's runner to run tests.
+
+You can run these tests with the command:
+
+`./pants test examples/tests/scala/org/pantsbuild/example/specs2`
+
+This target can also be imported in IntelliJ. If you choose to run it with IntelliJ Scala Runner, you can see a Specs2 run configuration generated under Run -> Edit Configurations.