Add an option for hosting a server.

[FlafyDev]

Right now all the data of the reader is saved on the device.
This includes the epub files of the books and the reading progress of each book.
The disadvantages of this approach:

1. Can't use the app on multiple devices without having to manually sync all the data and reading progress yourself.
2. Can easily reach a few hundreds of MBs of data after adding a few books (because each book's epub file takes like 10mb)
3. With big amounts of data it takes a while to create backups (zipping all the data).

For these reasons, it should be possible to connect to a server to take care of all these problem.