Remove server reporting, fixes for poltergeist

- Do not POST to /access_lint/errors. This should be a configuration
  option eventually.
- Log violation message in a format that poltergeist webdriver can
  handle.
- Check deep equality of the violation messages with underscore
  (JSON.stringify was failing in poltergeist).
- Update Readme.

Only log distinct new errors

- Check equality of stringified violations to see if we've already had a
  warning with that signature.

Run auditor on mutation events

- Re-run the audit when the DOM changes (via AJAX, or websockets, for
  example).
- Treat the console message as a warning rather than a log message.
- Reference the actual DOM node in the console warning, which makes it
  highlightable and clickable in developer tools.
- Does not rerun on visibility changes (yet).

Disable color contrast and remove version

- Color contrast checking makes the page scroll, which is not ideal in
  dev and staging.

Add eslint