AASM defines constants for each state name

So, I found I was hard coding state names in situations such as tests, or custom scopes. Hard coding such things is a bad thing, so I wrote this pull request to auto-define constants for each defined state.

Example:

```ruby
class Foo
  include AASM

  aasm do
    state :initialized
    state :calculated
    state :finalized
  end
end

> Foo::STATE_INITIALIZED
=> :initialized
> Foo::STATE_CALCULATED
=> :calculated
```