Fix iCRC type to make it 4-byte on both 32-bit and 64-bit platforms

sizeof(unsigned int) is 4 on both 32-bit and 64-bit platforms.
sizeof(unsigned long) is 4 on 32-bit platform and 8 on 64-bit one.

http://stackoverflow.com/questions/13335587/sizeof-unsigned-int-in-microsoft-x64-compiler