split the functionality for standalone Common
more changes in modules dependencies
accept a config block to configure p_a from initializer

it works before an instance of Config is present and provides
a simple DSL:

PublicActivity::Config.set do
  orm :mongoid
  enabled false
end
fix accessors for orm from Config
update readme to reflect database setup changes
mention the lightweight-standalone Common in readme
extract new role: deactivatable, use in Model
Merge branch 'master' of github.com:pokonski/public_activity into config-block
Merge branch 'master' of github.com:pokonski/public_activity into split_functionality
Merge branches 'split_functionality' and 'config-block'

merges #56 #57