add this.touch function to casper

One of My friends failed to use casperjs to write test case in mobile web page testing, because this web page use touch event to trigger a action, but capserjs only support click event. I wrote following codes to make touch event available in capserjs, and I think a touch event supporting in mobile web page testing is necessary, as the touch event in cellphone is very common. So I commit my patch back to capserjs, and hope it helps others.
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