Workaround for 64-bit MP4 atoms.

Workaround for the issue #411.
TagLib already supports large files on the systems that ```long``` is 64-bit. (e.g. GCC on 64-bit Linux, MacOSX etc.) However, ```MP4::Atom``` can't handle 64-bit atoms even on such systems.

This is just a workaround, so I think that we should change ```MP4::Atom::offset``` and ```length``` to ```long long``` or ```offset_t``` in ```taglib2``` branch.