Some code optimizations, minor fixes, and documentation corrections.

• Changed PMValidationUnit to use fast enumeration in the validateText method instead of the block form. Performance comparisons bear out that fast enumeration is the better choice for most reasonable use cases, and avoids concurrency issues.
• Changed all class constructors to use return instancetype instead of id.
• Fixed some NSInteger/NSUInteger mismatches.
• Updated comments to reflect this and some previous changes.