Implement Bazel build

To run the tests:

  bazel test //...

To build the Gerrit plugin API, run:

  bazel build gerrit-plugin-api:plugin-api_deploy.jar

To build the Gerrit extension API, run:

  bazel build gerrit-extension-api:extension-api_deploy.jar

TODOs:

Licenses
Reduce visibility (all public for now)
Generate HTML Documentation
Core plugins
gerrit_plugin() rule to build plugins in tree and standalone modes
GWT UI (only gwt_module() skylark rule is provided, no gwt_binary())
PolyGerrit UI
WAR
Publish artifacts to Maven Central
Ask Bazel team to add Gerrit to their CI on ci.bazel.io

Contributed-By: Han-Wen Nienhuys <hanwen@google.com>
Change-Id: I9a86e670882a44a5c966579cdeb8ed79b1590de3
diff --git a/tools/bzl/gwt.bzl b/tools/bzl/gwt.bzl
new file mode 100644
index 0000000..29987ef
--- /dev/null
+++ b/tools/bzl/gwt.bzl
@@ -0,0 +1,28 @@
+# Copyright (C) 2016 The Android Open Source Project
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+# http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+# GWT Rules Skylark rules for building [GWT](http://www.gwtproject.org/)
+# modules using Bazel.
+load('//tools/bzl:java.bzl', 'java_library2')
+
+def gwt_module(gwt_xml=None, resources=[], srcs=[], **kwargs):
+  if gwt_xml:
+    resources += [gwt_xml]
+  if srcs:
+    resources += srcs
+
+  java_library2(
+    srcs = srcs,
+    resources = resources,
+    **kwargs)