Use capybara-accessible driver by default.

capybara-accessible is a Selenium based webdriver that runs
accessibility assertions. I've tagged exiting failing examples as
inaccessible => true. We should avoid adding this tag to new features,
and venture to remove the existing ones.
Fix accessibility related to items views.

The items integration spec is now passing accessibility audits.
Removed the inaccessible tag, so that future regressions will raise
failures.

[#36]