preparing versions 3.1 and 4.0

AASM4.md

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+# Planned changes for AASM 4
+
+ * **done**: firing an event does not require `to_state` parameter anymore (closing issues #11, #58, #80)
+ * don't allow direct assignment of state attribute (see #53)
+ * remove old callbacks (see #96)
+ * remove old aasm DSL (like `aasm_state`)
+
+
+# Planned changes for AASM >= 3.1
+
+ * deprecate old aasm DSL (`aasm_state`, etc.)
+ * deprecate old callbacks

callbacks.txt

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+callbacks
+
+begin
+  old_state       exit                # old? should be deprecated -> use old_state.before_exit instead
+  event           before
+  old_state       before_exit
+  new_state       before_enter
+  new_state       enter               # old? should be deprecated -> use new_state.before_enter instead
+    ...update state...
+    transition    guard
+    transition    on_transition
+    event         success             # if persist successful
+  old_state       after_exit
+  new_state       after_enter
+  event           after
+rescue
+  event           error