Replacing `better_errors` with `web-console` beta.

`better_errors` presently has some issues with Ruby 2.1+; after seeing
a Reddit /r/ruby link to an enthusiastic mention of the `web_console`
beta, giving it a try. :-)

RSpec: 676 examples, 0 failures (but see Issue #87)
RuboCop: 151 files inspected, no offenses detected
PostUpdateValidator utility class/specs.

As described in #89, utility class to take a Post-like object and a set
of attributes with which to update it, and exposes a public API to
  1. Retrieve the combined data (`#combined_data`);
  2. Validate the combined data (`#valid?`); and
  3. Return a Hash of any error messages (`#messages`).

Do note that, as proven in the spec, `#messages` will always return an
empty hash i `#valid?` has not yet been called.

RSpec: 697 examples, 0 failures (but see Issue #87)
RuboCop: 153 files inspected, no offenses detected
Sometimes you want to validate *just* the object.

PostUpdateValidator now treats the `data` parameter to #initialize as an
optional parameter, defaulting to an empty hash. This will have the
effect of validating the current `post` (whether entity or ActiveRecord
model).

So why do we still have validation code in the model? That's not the
only, or even the most pressing, question at the moment.

RSpec: 697 examples, 0 failures (but see Issue #87)
RuboCop: 153 files inspected, no offenses detected
Merge pull request #91 from jdickey/issue-89

[Fix #89] PostUpdateValidator service and specs.