in Cocoa URL loader implementation, changed int datatypes to off_t datat...

NOTE: This is a 'duplicate' of https://github.com/cinder/Cinder/pull/434, because I forgot to use a separate branch for the pull request.

In Cocoa URL loader implementation, changed int datatypes to off_t datatypes to prevent int overflow when downloading large files. Without this fix, line 180-181 would overflow the int so the realloc on 182 would try to allocate a negative amount of space.