Mark designated initialisers with NS_DESIGNATED_INITIALIZER

[pbendersky]

This hints the compiler to give warnings if another init method, or subclass doesn't call the designated initializer.

[phatblat]

For reference on NS_DESIGNATED_INITIALIZER: https://developer.apple.com/library/ios/releasenotes/ObjectiveC/ModernizationObjC/AdoptingModernObjective-C/AdoptingModernObjective-C.html#//apple_ref/doc/uid/TP40014150-CH1-SW8

[phatblat]

Closing now that #449 is merged.