Article controller specs for '#show' now verify all model fields against fixture.
Added "new" action to (spec for) Article controller. This had several ripple effects:

- I finally read the Draper README more thoroughly, and discovered the #decorates_finders method. No more need for an ArticlesController#index override that just decorates the Articles.
- Added view and view spec for new article
- Added form partial which is rendered by the new-article view
- Added ArticleDecorator#status_options method (and spec), used by the form partial
- Added ApplicationHelper#explain_errors method (and spec). In ApplicationHelper since it's not specific to a model (future Contribution forms will use it, too).
- Added simple_form Gem to Gemfile; it's used by the form partial.