deprecate use of json page source

lib/devices/android/android-controller.js

@@ -17,6 +17,7 @@ var errors = require('../../server/errors.js')
   , path = require('path')
   , XMLDom = require("xmldom")
   , helpers = require('../../helpers.js')
+  , warnDeprecatedCustom = helpers.logCustomDeprecationWarning
   , warnDeprecated = helpers.logDeprecationWarning;
 
 var androidController = {};
@@ -371,6 +372,14 @@ androidController.getPageSource = function (cb) {
     ],
     // Top level cb
     function () {
+      warnDeprecatedCustom('page source', 'json',
+        "You got the source of the app using the page source command. This " +
+        "command is undergoing a large change which will be released in " +
+        "Appium 1.0. The change will include returning an XML document " +
+        "instead of a JSON object. If you depend on the page source command " +
+        "you should try the new version, which is accessible by doing " +
+        "`driver.execute_script('mobile: source', [{type: 'xml'}])`, or " +
+        "the equivalent in your language bindings.");
       cb(null, {
         status: status.codes.Success.code
       , value: json