Longint

I am starting to clean up mym. Are you guys ok to merge the longint branch into master?
I have modified it to correctly deal with signed and unsigned 64-bit ints. There is a matching commit in datajoint to insert 64-bit ints from Matlab without loss of precision: https://github.com/datajoint/datajoint-matlab/commit/b4e9c826be9a9adf53f68d3f8c6541b69b7f9f92

The only concern is the handling of NULL. Integer datatypes do not have "NaN", hence MySQL NULL is currently mapped to 0 to mimic Matlab's behavior: `int64(NaN) = uint64(NaN) = 0`