Added support for short keys; Addon modified with nan

I have made some changes:
* Addon now performed via [nan](https://github.com/rvagg/nan) layer, (tested for `node-gyp` versions: `0.13.1`, `0.12.2`, `0.11.0`, `0.10.10`, `0.8.5`)
* Key length perfomed as in php `mcrypt_encrypt`. Now function pads the key up to the nearest valid key length (16, 24, or 32 bytes) with `\0` symbol.