emacs: add launchd plist for server daemon.

Since emacs version 23.1 the emacs executable accepts a --daemon flag
which will start emacs as a server daemon. You can then connect to the
server using emacsclient (reducing startup time etc).
See http://www.emacswiki.org/emacs/EmacsAsDaemon#toc7

Formula/emacs.rb

@@ -114,6 +114,25 @@ class Emacs < Formula
     end
   end
 
+  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>Label</key>
+      <string>#{plist_name}</string>
+      <key>ProgramArguments</key>
+      <array>
+        <string>#{opt_bin}/emacs</string>
+        <string>--daemon</string>
+      </array>
+      <key>RunAtLoad</key>
+      <true/>
+    </dict>
+    </plist>
+    EOS
+  end
+
   test do
     assert_equal "4", shell_output("#{bin}/emacs --batch --eval=\"(print (+ 2 2))\"").strip
   end