Add events availability methods

[dapi]

No description provided.

[geekq]

Thanks for your contribution! Please note, that workflow lib is currently under
heavy refactoring preparing for the 2.0 release where ActiveRecord support is
extracted into a separate gem. Once 2.0 is released, we can work on integrating
your extension.

What I can already see:

• Note about motivation for this extension would be helpful! (what this feature can be used for?)
  Like "showing buttons in the GUI for all the available transitions"?
• It can be also provided as a unit test - unit tests covering the change are required anyway
• both functions have got very similar implementation - probably merge them into one,
  if state parameter is not provided - use the current state