Implement File history

[nyxxxie]

It is desired that File (see #26) should be able to store a history of its changes so that a user may undo/redo changes they may have made. This is being implemented at the library level since the spade library is primarily designed to be used with another client editor application (means less code for client to write and allows for the client to be remote). Simple linear history model may be implemented, but a tree-based model is preferred.