added getAccessToken API to iOS

### Test Code

```
document.addEventListener('deviceready', function () {
	var fbLoginSuccess = function (userData) {
	    alert("UserInfo: " + JSON.stringify(userData));
	    facebookConnectPlugin.getAccessToken(function(token) {
	        alert("Token: " + token);
	    }, function(err) {
	        alert("Could not get access token: " + err);
	    });
	}
	
	facebookConnectPlugin.login(["basic_info"], 
	    fbLoginSuccess, 
	    function (error) { alert("" + error) }
	);	            
}, false);
```