Introduce different write modes

[medikoo]

In first place it'll be great to have possibility to have write mode locked. So any unattended changes are rejected.

Then it might be good to have a method which will allow to proceed with updates in provided callback, e.g.:

database.write(function () {
  obj.foo = 'bar'; // change saved
  obj.delete('bar'); // change saved
});
obj.foo = 'foo'; // exception thrown

Above could also serve as transaction handler (see also #58 for notes on transactions handling)

Then it might be good to have two different write methods:

1. Event records are created for all update attempts (even if property values didn't change). so e.g.:

  obj.foo = 'bar'; // event record created
  obj.foo = 'bar'; // event record created

2. Only when property value is changed event record is created

  obj.foo = 'bar'; // event record created only if value was not 'bar' already
  obj.foo = 'bar'; // no event record created