Add new command to press on the MENU button for Android.

The Android UiDevice has method to press the MENU button. To support this a new mobile command is added 'pressMenu'.

For python binding, the client code will be...

```python
self.driver = webdriver.Remote('http://localhost:4723/wd/hub', desired_caps)	
self.driver.execute_script("mobile: pressMenu")
```