GIF search plugin

This is a simple gif plugin using <a href="https://github.com/giphy/GiphyAPI">Giphy API</a>, similar to what we had in phenny.

Example:
<code>21:26 <@kesara> !gif cat</code>
<code>21:26 < gerri> http://media.giphy.com/media/YAxCzfAc7MHPa/giphy.gif</code>