Add driver method

Add screenshot method
Add attr readers

lib/appium_lib/driver.rb

@@ -34,6 +34,9 @@ module Appium
   class Driver
     @@loaded = false
 
+    attr_reader :app_path, :app_name, :app_package, :app_activity,
+                :app_wait_activity, :sauce_username, :sauce_access_key,
+                :port, :os, :ios_js
     def initialize opts={}
       opts = {} if opts.nil?
       # Path to the .apk, .app or .app.zip.
@@ -158,6 +161,15 @@ def restart
       start_driver
     end
 
+    # return driver
+    def driver
+      @driver
+    end
+
+    def screenshot png_save_path
+      @driver.save_screenshot png_save_path
+    end
+
     # Quits the driver
     # @return [void]
     def driver_quit