Add script for GifGif

The kind folks at GifGif recently added an API. More information available on [their site](http://gifgif.media.mit.edu/data).

This script aims to return a gif based on an emotion:
```
[ "amusement", "anger", "contempt", "contentment", "disgust", "embarrassment", "excitement", "fear", "guilt", "happiness", "pleasure", "pride", "relief", "sadness", "satisfaction", "shame", "surprise" ]
```
I opted to _not_ use the `robot.respond`, rather having Hubot listen for the user to issue `gifgif (emotion)` instead.