#71 Replace __int64 with long long type

[Kimmo Varis]

We've been using __int64 type as 64-bit integer type. However it is MS compiler specific type.

There is standard long long type which is also 64-bit integer. But earlier we could not use long long type as the VC6 compiler did not know it. So we had no other choice than using __int64 for VC6 compatibility.

Now that we aren't supporting VC6 anymore we can start using long long and convert existing __int64 usages to long long.