Mark cache dirty on any update

[eniklas]

Currently, the cache is marked dirty when an API call is attempted, which is used to determine if the cache has been updated since we started up or last saved it, in which case it needs to be saved to disk again. The dirty status is a value in the cache data itself, which isn't great. A better way to do this might be to make the cache or the cache data a subclass of dict, with a dirty attribute that gets set to True on __setitem__ or __delitem__. Some attempts at this were made in #43 but I couldn't get it to work before my patience ran out.