Some refactoring, warden manager configuration is overridable

Appart from some organization refactoring like moving session controller actions into a separate file and moving app default settings out of helpers module and into Warden.registered method, warden manager configuration can be overriden from padrino app:

    set :warden_config do |manager|
      manager.scope_defaults :api, strategies: [:token], store: false
    end

Thus the default scope can be changed or more default strategies can be added for single or multiple scopes.

Changed unauthenticated path to /unauthenticated instead of /sessions/unauthenticated, conforming with warden default fail action, thus there is no need to set a custom action in scope_defaults declaration.

Not overriding http method on failure, instead defining an action by each of the http methods supported by sinatra, overriding http to POST seemed a bit hackish to me and I got some issues with crfs token, con is that now a lot of actions are defined. A better alternative would be a failure app perharps (but then padrino render has to be implemented).

Merged icco/patch-1 fixing markdown documentation.