1. add background_app
1. add bundleid
1. add udid
fix for https://github.com/jollychang/robotframework-appiumlibrary/issues/7

```
In [10]: from appium import webdriver
In [11]: app = '/Users/jollychang/tmp/DoubanDaily-iOS/DoubanDaily/build/Release-iphonesimulator/DoubanDaily.app'
In [12]: driver = webdriver.Remote(
    command_executor='http://127.0.0.1:4723/wd/hub',
    desired_capabilities={
        'browserName': '',
        'device': 'iPhone Simulator',
        'platform': 'Mac',
        'version': '7.0',
        'app': app,
        'bundleid': "com.douban.daily-inhouse"
    })
```

```
info: Responding to client with success: {"status":0,"value":{"webStorageEnabled":false,"locationContextEnabled":false,"browserName":"","platform":"Mac","javascriptEnabled":true,"databaseEnabled":false,"takesScreenshot":true,"warnings":{},"desired":{"app":"/Users/jollychang/tmp/DoubanDaily-iOS/DoubanDaily/build/Release-iphonesimulator/DoubanDaily.app","platform":"Mac","browserName":"","version":"7.0","device":"iPhone Simulator","bundleid":"com.douban.daily-inhouse"},"app":"/Users/jollychang/tmp/DoubanDaily-iOS/DoubanDaily/build/Release-iphonesimulator/DoubanDaily.app","version":"7.0","device":"iPhone Simulator","bundleid":"com.douban.daily-inhouse"},"sessionId":"ffa01fd3-8d08-4044-8c7c-402da19311b1"}
```