Support for installing rdb-tools under io.js (possibly also Node 0.12)

[assaf]

This PR contains changes we needed to build rdb-tools with io.js. It should also apply to Node 0.12, but we only tested our fork against io.js (2.2.x and later).

• Rewrote native code using nan, to abstract some of the changes between Node 0.10/0.12 and io.js. No change in functionality, API, or CRC algorithm.
• Changed dependencies to use int64-native-node12, a fork of int64-native that builds on Node 0.12/io.js
• Changed dependencies to use our fork of lzf, which builds on io.js ((there's a separate PR to merge those changes back into lzf)

[codeaholics]

Thanks for this. I've not started looking into io.js (or Node 0.12) yet. But I notice this PR fails on Travis. So my questions are...

1. Can you make it work on Travis, please?
2. Is there a way to unify the code such that it works on Node < 0.12, and io.js and Node 0.12+ ?

[assaf]

This PR uses nan, so the code should build on all versions of Node 0.8 or later, and all versions of io.js.

int64-native will not install on 0.12 or later, and the alternative int64-native-node12 seem to be written for 0.12 or later only (won't install on 0.10).