New xkcd module

**...because there is always a relevant XKCD comic for every situation in life!**

It can either give the most recent XKCD comic or can also give top 3 most relevant comic results if supplied with a query in the form of a sufficiently descriptive sentence

For example, 
```!xkcd``` returns ```March of the Penguins : http://xkcd.com/1408```  and ```!xkcd teach your kids linux``` returns the following : 
```
20:16:56   WololoPiBot | Cautionary : http://xkcd.com/456
20:16:58   WololoPiBot | Linux User at Best Buy : http://xkcd.com/272
20:17:01   WololoPiBot | That Lovin' Feelin' : http://xkcd.com/317 
```
Observe that the top one is most relevant. It uses queries to backend of ```http://relevantxkcd.appspot.com/``` which calculates the relevant xkcd based on some smart machine learning stuff done using explainxkcd.com wiki data.