Ensure fileData is actually copied when reading uncompressed zip files

On pre-10.9 systems, [NSData copy] returns the same object, see rdar://14670403. Using [NSData dataWithBytes:length:] always creates a new object.

Create data objects using "alloc"/"init" instead of "data" to avoid autorelease pools not getting drained quickly.

Minor style fix.
Merge pull request #63 from goetzf/master

Fix memory corruption when reading uncompressed archives on pre-10.9 systems.