Add task for updating documentation

This task will update the documentation in the gh-pages branch.
The master branch must be clean for this to work.

.gitignore

@@ -1,3 +1,6 @@
 coverage
 doc
 keyfile.json
+
+# directories left by gh-pages
+_site

rakelib/gh-pages.rake

@@ -0,0 +1,39 @@
+# Copyright 2014 Google Inc. All rights reserved.
+#
+# 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.
+
+namespace :pages do
+  desc "Updates the documentation on the gh-pages branch"
+  task :master do
+    branch = `git symbolic-ref --short HEAD`.chomp
+    if "master" != branch
+      puts "You are on the #{branch} branch. You must be on the master branch to run this rake task."
+      exit
+    end
+
+    commit_hash = `git rev-parse --short HEAD`.chomp
+
+    puts `git checkout gh-pages`
+    puts `git rm -rf docs/master`
+    puts `git commit -am "Update documentation for #{commit_hash}"`
+    puts `git checkout #{branch}`
+    puts `rake docs`
+    puts `mkdir docs`
+    puts `mv doc docs/master`
+    puts `git add docs/master`
+    puts `git checkout gh-pages`
+    puts `git add docs/master`
+    puts `git commit --amend --no-edit`
+    puts `git checkout #{branch}`
+  end
+end