Simplest-possible Pundit policy for PostData authorisation.

Note that:
* This isn't going to live very long. The "Guest User" and "Registered
  User" are domain concepts, and thus should be handled in entities and
  DSOs.
* This really *is* apparently all that's needed for now. THe default,
  generated `ApplicationPolicy` class locks everything down; subclasses
  mostly relax things as appropriate.
* Still not clear on if `#index` needs to be explicitly authorised; that
  will be answered by the next commit.

Pinging Issue #34 and Issue #1.

RSpec: 3513 examples, 0 failures
RuboCop: 101 files inspected, no offenses detected
Starting to integrate Pundit-based PostDataPolicy into app.

Hilarity ensued, of course.

* Added Pundit recommended error-notification method to
  `ApplicationController` (which will eventually uncomment the
  commented-out filters);
* New support class `FeatureSpecLoginHelper` provides an easy-to-use,
  feature-spec DSL-using (actually, Capybara DSL-using) means to
  register a new user and log that user in or out;
* `PostsController` is the only controller yet supporting Pundit
  authorisation;
* Spec for `PostsController` verifies the `#new` and `#create` actions
  for a registered user and for the Guest User;
* All Post-related feature specs updated to create and log in a
  registered user;
* User login/logout feature spec updated to use login-helper support
  class.

Pinging Issue #34 and Issue #1.

RSpec: 363 examples, 0 failures
RuboCop: 102 files inspected, no offenses detected
Pundit authorisation of BlogController with BlogData model now working.

Along for the ride:
* Spec coverage of ApplicationPolicy, bringing us back to 100% coverage;
* Updated to-be-uncommented filters in ApplicationController to match
  Rails 4 usage;
* BlogDataDecorator tells Pundit that objects it decorates should be
  authorised using the BlogDataPolicy; Pundit usage ties policy class
  names to model class names;
* BlogController spec now satisfies Pundit authorisation.

RSpec: 379 examples, 0 failures
RuboCop: 105 files inspected, no offenses detected
Pundit authorisation of UserController with UserData model now working.

RSpec: 380 examples, 0 failures
RuboCop: 107 files inspected, no offenses detected
Final bit of Step 2 of the 'latest...strategy' from Issue #34.

See https://github.com/jdickey/new_poc/issues/34#issuecomment-49047556
for the spec; basically a '"traditional" implementation of Pundit as a
first step to what's coming, and to prove out what to me is new tech.'
This was a *large* brick exiting the orifice laterally, and cleanup
touched all the other authentically resource-based controllers that had
already been adapted. The kicker, though, was getting login and logout
to behave enough like a RESTful resource that Pundit, and ActiveModel,
could deal with it. Login (#create) and logout (#destroy) were
predictably gnarly (in the "earn"-while-you-learn sense); what broke the
feature specs at the very end (and thus the app in a browser) was the
fact that there needed to be a #new action, properly authorised, in
order to get to #create anything. Which makes perfect sense -- in
hindsight. :anguished: Oh, well; the stated objective WAS to learn the
new-to-me tech.

RSpec: 389 examples, 0 failures
RuboCop: 111 files inspected, no offenses detected
Pundit specs reworked per now-bundled RSpec support.

Also locked to a specific commit of RuboCop until recent problems solved
with last couple of updates.

RSpec: 389 examples, 0 failures
RuboCop: 110 files inspected, no offenses detected
A challenge and a conundrum, moving Pundit into DSOs.

This should be fully documented in the first ~60 lines of
SessionsController; the reader is invited to peruse the (uncommented)
code in the controller's #new method and compare it to the #execute
method of the DSO, while keeping in mind the linked code submitted as a
PR for Pundit (adding PunditAuthorizer).

On a related track, if anyone can explain to me why the Obvious Code for
what is now lines 26-27 does *not* Just Work, and even better, how to
have a less egregious hack there, I would greatly appreciate it.

RSpec: 389 examples, 0 failures
RuboCop: 110 files inspected, no offenses detected
SessionsController#new no longer uses DSO.

Pundit authorisation code works with the stock Gem.

RSpec: 395 examples, 0 failures
RuboCop: 110 files inspected, no offenses detected
Merge pull request #46 from jdickey/pundit-1

Pundit authorisation added to app. Closes #34. Closes #32. Closes #43.