Ported r2822 from ZXing

Original commit message: "AztecWriter improvements from fyellin"
Rename ZXEncoder to ZXQRCodeEncoder
Add Objective-C wrappers around BOOL and int arrays
Update comments to better match Javadocs from ZXing

Also rename constants to include "ZX_" prefix
Various speed improvements

Most of these are due to removing unnecessary method calls for
properties.