Adding support to multiple docsets query

Hello! This pull request allows searching on multiple docsets.

I tried to put the logic of everything related to the query within the ZealSearchQuery class and not in ZealDocsetsRegistry. Basically, what has changed is that instead of storing a string, the system will store a list of strings, used to determine on search if the docset should be considered in the results. 

Also took the opportunity to update the readme with information that I found useful, especially some additional packages I needed to install in order to compile the project. 

By the way, great project! I use Dash on mac, and I hope this contribution will help others like me that use linux either! 

ps: I'm also creating a plugin for IntelliJ, much based on what Kapelli created, to allow Zeal integration, how can I put it in the http://zealdocs.org/usage.html#plugins page?

Here's a preview of my changes:
![zeal-docsets](https://cloud.githubusercontent.com/assets/1536962/4619539/35a557f6-5316-11e4-9c39-cfd768a2fcd2.png)