http://appium.io/slate/en/master/?python#finding-and-interacting-with-elements
https://developer.apple.com/library/ios/documentation/Cocoa/Conceptual/Predicates/Articles/pSyntax.html#//apple_ref/doc/uid/TP40001795-215868
http://appium.io/slate/en/master/?python#ios-predicate
IOS app元素定位方式：
- ios uiautomation（ios-only）：用的是IOS 的谓词查找，类似于xpath的谓词定位
-android uiautomator(Android-only)
accessibiliy id: 相当于id/name定位
具体用法查看：
http://appium.io/slate/en/master/?python#ios-predicate
https://developer.apple.com/library/ios/documentation/Cocoa/Conceptual/Predicates/Articles/pSyntax.html#//apple_ref/doc/uid/TP40001795-215868