Allow callback to key save function

[chapel]

For a single key, it would make sure the key was saved before calling run.

For multiple keys, it would internally count the callbacks and when all saves have been completed, finally call run.

[chapel]

Due to #5 this really is not needed. missed.save() isn't actually writing directly, but compiling the array that will then be sent to redis in a multi command save. So essentially, it doesn't save until done() is called anyways.