Exclude org.hamcrest from the shading rules for junit runner

The shading of the junit runner specifically excludes the org.junit classes.  As it turns out, we also need to exempt the org.hamcrest classes so that APIs work correctly.

Testing Done:
CI is running at: https://travis-ci.org/pantsbuild/pants/builds/60578769

We use 'assertThat' a fair amount in our code base.  This change fixes those problems.

Bugs closed: 1466, 1467

Reviewed at https://rbcommons.com/s/twitter/r/2137/

src/python/pants/backend/jvm/tasks/junit_run.py

@@ -99,7 +99,10 @@ def register_options(cls, register, register_jvm_tool):
                       # Clearly both tests and the runner need access to the same @Test, @Before,
                       # as well as other annotations, but there is also the Assert class and some
                       # subset of the @Rules, @Theories and @RunWith APIs.
-                      custom_rules=[Shader.exclude_package('org.junit', recursive=True)])
+                      custom_rules=[
+                        Shader.exclude_package('org.junit', recursive=True),
+                        Shader.exclude_package('org.hamcrest', recursive=True)
+                      ])
 
   def __init__(self, task_exports, context):
     self._task_exports = task_exports

testprojects/tests/java/org/pantsbuild/testproject/matcher/BUILD

@@ -0,0 +1,10 @@
+# Copyright 2015 Pants project contributors (see CONTRIBUTORS.md).
+# Licensed under the Apache License, Version 2.0 (see LICENSE).
+
+
+junit_tests(name='matcher',
+  sources=['MatcherTest.java'],
+  dependencies=[
+    '3rdparty:junit',
+  ],
+)

testprojects/tests/java/org/pantsbuild/testproject/matcher/MatcherTest.java

@@ -0,0 +1,47 @@
+// Copyright 2015 Pants project contributors (see CONTRIBUTORS.md).
+// Licensed under the Apache License, Version 2.0 (see LICENSE).
+
+package org.pantsbuild.testproject.matcher;
+
+import org.hamcrest.BaseMatcher;
+import org.hamcrest.Description;
+import org.junit.Test;
+
+import static org.hamcrest.CoreMatchers.containsString;
+import static org.junit.Assert.assertFalse;
+import static org.junit.Assert.assertThat;
+import static org.junit.Assert.assertTrue;
+
+/**
+ * This test insures that the junit and hamcrest classes work for a junit test.
+ */
+public class MatcherTest {
+
+  class FooMatcher extends BaseMatcher<String> {
+
+    @Override public boolean matches(Object o) {
+      assertTrue(String.class.isInstance(o));
+      String value = (String) o;
+      return value.contains("foo");
+    }
+
+    @Override public void describeTo(Description description) {
+      description.appendText(this.getClass().getSimpleName());
+    }
+  }
+
+  @Test
+  public void testMatcher() {
+    FooMatcher matcher = new FooMatcher();
+    assertTrue(matcher.matches("foobar"));
+    assertFalse(matcher.matches("Hello World!"));
+  }
+
+  /**
+   * This test fails if the org.hamcrest classes are shaded in junit runner
+   */
+  @Test
+  public void testAssertThat() {
+    assertThat("Give me some food", containsString("foo"));
+  }
+}