Enabled Performance Logs for iOS

Added performance logs for iOS, similar to what Chromedriver does. 

Tested using on both mobile web and hybrid (cordova) app 
```javascript
var wd = require('wd');

var b = wd.promiseRemote('http://localhost:4723/wd/hub');
var caps = {
	"platformName": "iOS",
	"platformVersion": "7.1",
	"browserName": "", //"Safari",
	"deviceName": "iPhone Simulator",
	"app": "/sample/platforms/ios/build/emulator/HelloCordova.app.zip",
	"bundleId": "io.cordova.hellocordova",
	autoWebview: true,
	loggingPrefs: {
		performance: 'ALL'
	}
};

b.init(caps).then(function() {
	return b.log('performance');
}).then(function() {
	return require('Q').delay(1000);
}).then(function() {
	return b.log('performance');
}).then(function(data) {
	console.log(data);
}).fin(function() {
	return b.quit();
}).done();
```
I see that Android supports `enablePerformanceLogging`, but the selenium standard (and Chromedriver) seem to call this `loggingPrefs`. This way, existing chromedriver code need not change.