Adopting enumeration marco for PMKBlockDesciptionFlags

As per
https://developer.apple.com/library/ios/releasenotes/ObjectiveC/Moderniz
ationObjC/AdoptingModernObjective-C/AdoptingModernObjective-C.html#//app
le_ref/doc/uid/TP40014150-CH1-SW6
Merge pull request #14 from mattmorton/master

Adopting enumeration marco for PMKBlockDesciptionFlags