zookeeper: add launchd plist

Formula/zookeeper.rb

@@ -120,4 +120,32 @@ class Zookeeper < Formula
     cp 'conf/zoo_sample.cfg', 'conf/zoo.cfg'
     (etc/'zookeeper').install ['conf/zoo.cfg', 'conf/zoo_sample.cfg']
   end
+
+  plist_options :manual => "zkServer start"
+
+  def plist; <<-EOS.undent
+    <?xml version="1.0" encoding="UTF-8"?>
+    <!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
+    <plist version="1.0">
+      <dict>
+        <key>KeepAlive</key>
+        <dict>
+          <key>SuccessfulExit</key>
+          <false/>
+        </dict>
+        <key>Label</key>
+        <string>#{plist_name}</string>
+        <key>ProgramArguments</key>
+        <array>
+          <string>#{opt_bin}/zkServer</string>
+          <string>start-foreground</string>
+        </array>
+        <key>RunAtLoad</key>
+        <true/>
+        <key>WorkingDirectory</key>
+        <string>#{var}</string>
+      </dict>
+    </plist>
+    EOS
+  end
 end