Fixes xib crash

Fixes a xib crash caused by use_frameworks!

Merged upstream from homekuanyi

Hopefully this fixes the crash cause by trying to load a Xib called "AXStretchableHeaderTabViewController"

Updated podspec

Updated podspec. Hoping this will make the most recent release to get pulled by 'pod install'

Changes to make bar styleable

Exposing some things to subclasses to allow for styleable bar.

Fixed stupid crash

[scrollView removeObserver:self forKeyPath:@"contentOffset"]; was crashing A TON cuz it's not safe.

I fixed by adding if (scrollView && scroll.observationInfo) before it