Add support for app extensions

[PatrickNLT]

WYPopover currently doesn't compile on iOS extensions.

A workaround is to wrap every forbidden call into a #if !defined(WY_APP_EXTENSIONS) macro. This is used in most libraries.

I've been using this fix in production for a few weeks already, it works very well.

[PatrickNLT]

Would you consider merging this pull request?

[vitalys]

@PatrickNLT I'm worried about using UIInterfaceOrientationPortrait for orientation in case of an app extension. How does it work on iPad?