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).

The file provided by @flashanand is here: https://testanandsiva.sharefile.com/d/s6c01695aeb142eaa
I tested the file on my 64-bit Linux box and could read/write the tag properly.
I hope somebody would test the file on MacOS X. 
I think no, because we have this check. 
Additionally, the ```longLength >= 8``` check shows the confusing message ```"MP4: 64-bit atoms are not supported"``` when ```longLength < 8```.