Add simple back-end for self-hosted scenario

[MartinSGill]

In response to #59

Add a simple server-side script that can send/receive the todo.txt file.
Add a storage provider that calls this script to load/save the todo.txt file.

This may require adding a config file of some kind to easily set which storage providers are to be used when the page loads.

[MichaelTunnell]

I would love to get this feature. The only thing that stops me from using this project is the dependency on Dropbox. I can't stand Dropbox so I don't want to rely on such a service.

I'm not sure if this (#60), #61 or perhaps maybe integrating Syncthing from http://syncthing.net would be best but this is something I'd love to see be added.

[MartinSGill]

It works without dropbox, storing in local browser storage. But dropbox is used for syncing across platforms. Problem is that it needs to run in a browser, entirely in javascript and finding libraries is not easy.

The code's expandable, so adding a new load/save provider shouldn't be too hard. Happy for someone to help out.