Switch to __has_include marco

As CocoaPods 0.34 will be released soon, reading `COCOAPODS_POD_AVAILABLE_*` in `Pods-environment.h` is no longer supported, and using `#if __has_include(<UI7Kit/UI7AlertView.h>)` is [recommended](https://github.com/CocoaPods/CocoaPods/issues/1068#issuecomment-55536750) instead.

I also found UI7SearchView and UI7PickerView imports are missing in current implementation. Not sure if that is expected behavior but I added them back.

This closes #147.