Allow overwriting of events, can be very useful with inheritance

Hi,

First, thanks for this gem, it helps a lot, that's some very good stuff you made here :)

I had a problem with inheritance, I wanted an event in a child class to have different transitions than it's parent version.

Here's an example

````ruby
class Parent
  include AASM

  aasm do
    state :foo
    state :bar
    state :baz

    event :some_event do
      transitions :from => [:bar]
    end

  end

end

class Child < Parent
  aasm do
    state :other

    event :some_event do
      transitions :from => [:bar, :other]
    end
  end
end
````

Without this patch ``some_event`` won't be able to get called from ``:other``

What do you think ?