Fix Issue 46

[neurolabusc]

Fixes #46.

• Int64 values were read as 8 × int8, rather than 8 × uint8. Any value greater than 127 caused an overflow.
• Int64 values did not correctly read sign.
• Int64 values ignored littleEndian so failed for big-endian data.
• The reader name has been changed from getLongAt() to getInt64At() to be explicit (in C longs are only ≥32 bits).