Work with two states: finished, in_progress

#### This PR contains the following changes:

*   `Work` model now has states via the **[state_machine](https://github.com/pluginaweek/state_machine)** gem
*   *[set_finished](https://github.com/WonderCsabo/rails-for-business/blob/master/lib/tasks/set_finished.rake)* rake task which marks all existing ```Work``` as *finished*
*   *[set_one_time](https://github.com/WonderCsabo/rails-for-business/blob/master/lib/tasks/set_one_time.rake)* rake task which marks all existing *business* `Task` as *one_time*
*   [updated tests](https://github.com/WonderCsabo/rails-for-business/blob/master/spec)
*   **[FinishWork](https://github.com/WonderCsabo/rails-for-business/blob/master/lib/rails_admin_finish_work.rb)** custom action for *RailsAdmin*, which can set a ```Work˙˙˙ to be *finished*