Adds ability to select different adapters via a `workflow_adapter` class method.

[damncabbage]

This PR:

• Makes the ActiveRecord adapter more self-contained (by having it handle its own inclusion, extension and before_validation hook setup).
• Has Workflow look for a workflow_adapter class method first before going on to include ActiveRecord or Remodel adapters.

From the included documentation change:

Other adapters (such as a custom ActiveRecord plugin) can be selected by adding a workflow_adapter class method, eg.

class Example
  def self.workflow_adapter
    MyCustomAdapter
  end
  include Workflow
  # ...
end

(The above will include MyCustomAdapter instead of Workflow::Adapter::ActiveRecord.)

[damncabbage]

Where am I going with this? I have an app-specific ActiveRecord adapter I'm working on (to handle record-locking to avoid race conditions, among other things), but the auto-inclusion in lib/workflow.rb prevents me from using it without forking the project entirely.

(I'm happy to have this closed if you don't feel it fits with the direction of the project.)

[geekq]

yes, this is the direction I would like to go too - enable custom AR persistence.

Will review after my computerless ;-) holiday - in a week...

[damncabbage]

Got it; thanks. :)

[geekq]

This is exactly the way I would implement the possibility for different persistent behaviours, e.g. persist new workflow state immediately on bang-method call (default) vs. later on save call. But never found time for that ;-) - thank you very much!

Also solves #77 - custom ActiveRecord adapter (persisting on save call) would help.