v0.4.0

• The ISaveable interface Guid property ISaveable.Guid has been replaced with a string property called ISaveable.Key. This is a breaking change, but it enables the option to use either a unique string key or store a Guid as a string, rather than using Guids and serialized byte arrays exclusively.
• Added some much needed error checking to validate ISaveable keys when loading data. Previously, if a key was not found in the registered saveables dictionary, it would throw an unhandled exception.