Added page.post(...) function

Added page.post function to allow a render through a post action.

page.post(url, data[, callback])
	url : where to post
	data : html encoded string with the body of the request 
	callback : callback function (optional)

Example :
var phantom=require('node-phantom');
phantom.create(function(error,ph){
	ph.createPage(function(err,page){
		var data = "email=test@domain.com&pass=my_pass";
		page.post('http://domain.com/login',data,
		function(err,status){
			//Do stuff here
		});
	});
});