Use node-int64 for 64-bit long integers. Closes GH-1

Changes longs to return a [node-int64](https://github.com/broofa/node-int64); this module seems to be a good solution for 64-bit values as it supports exact, clamped, and rounded output representations. Fixes GH-1 better than the approximate solution in GH-8.

Also updated unit tests and they all pass :)