Standardize property/ivar usage

1. Synthesize ivars with leading underscores as is now conventional; and
2. Do not use accessor methods in initializer methods (see [Don’t Use Accessor Methods in Initializer Methods and dealloc](https://developer.apple.com/library/mac/documentation/Cocoa/Conceptual/MemoryMgmt/Articles/mmPractical.html#//apple_ref/doc/uid/TP40004447-SW6) in the _Advanced Memory Management Programming Guide._