Add DjVu support

> DjVu is a computer file format designed primarily to store scanned documents, especially those containing a combination of text, line drawings, indexed color images, and photographs. [...] DjVu has been promoted as an alternative to PDF, promising smaller files than PDF for most scanned documents.

[source Wikipedia](http://en.wikipedia.org/wiki/DjVu)

We support PDF, why not an open source alternative?

more info about DjVu on their [official website](http://www.djvu.org/).