Rename StateEvent model to StateChange to prevent Rails 3.2 conflict

Rails 3.2 added a GeneratedFeatureMethods module to AR classes which contains a state_events method. This conflicts with the one Spree had defined. Therefore, we will change ours to StateChange/state_changes to prevent this conflict.

Change-Id: I4f87f582b4c8a49f9cb8cd21b08df4e126cb93e8