add peer-to-peer file sharing plugin ZSync

Short intro:

ZSync is a peer-to-peer file sharing plugin that synchronize document files from one Koreader device to another in LAN environment. With the [ØMQ](http://zeromq.org/) framework it connects two or more Koreader devices directly in local network. 

If two Koreader devices are covered in the same WIFI router, one device can `publish` current viewing document in ZSync and other Koreader devices could `subscribe` that document in ZSync. Users needn't care about the IP address of their devices, they are just smart enough to find each other and transfer the document file.

 Currently only the document file is shared in ZSync but it will be easy to add support of sharing document history file and synchronize reading progress and settings among multiple Koreader devices. 

![screenshot from 2014-06-23 22 39 28](https://cloud.githubusercontent.com/assets/751535/3359055/68e5e96c-fae4-11e3-9cb5-93840e55454d.png)
![screenshot from 2014-06-23 22 40 47](https://cloud.githubusercontent.com/assets/751535/3359058/718fa5b2-fae4-11e3-9ed2-b98b87ebdd89.png)
so we don't call exitReader now?