Fix crash when using 2.3.1 in UICollectionViews or similar

[timbodeit]

In certain cases (collection views just being one of them), view initialization takes place within a layoutSubviews call.

-[UIView layoutSubviews] does some evaluation of constraints and seems to keep its own references to them. Therefore it is possible, that constraints of the placeholderView are being used, after we're done with view initialization.

As the placeholderView gets replaced it would previously be deallocated immediately. As NSLayoutConstraints only keep weak references on their items this lead to a dangling pointer on the firstItem property.

This change asynchronously dispatches a block enclosing the placeholderView. This defers the deallocation of the placeholderView until the end of the current runloop cycle and allows the current layoutSubviews call to go through before.

Based on the suggestion by @fuaiyi

Fixes #29

Might fix other issues regarding the iOS 10 Update.

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@sunnyxx Thanks for the English readme and in-code documentation.

Note to other users of the framework: Please keep GitHub communication (issues and comments) for open-source projects in English. English is not my mother tongue either, but using a common language makes collaboration a lot easier.