Adding 64bit int support

Hi
This adds support for longs (q and Q).

I came across jspack while working with the JS implementation of MAVLink, a UAV/drone communication library (https://github.com/mavlink/mavlink). MAVLink uses 64bit integers for timestamps (and for other stuff), so in order to use those I needed this. I added unit tests and Travis-CI support. You probably want to integrate your repository with Travis as well and change the link for the build status: https://github.com/AndreasAntener/node-jspack/compare/birchroad:master...master#diff-04c6e90faac2675aa89e2176d2eec7d8R4

Also it's "field" tested. I'm using this to parse and pack all my UAV telemetry links.

It would be great if you could update your NPM package with this.

Cheers
Ada