Add `redirect_url` config option

* Clarify in README that the config example shows the default values.
* Wrap long line at 80 characters.
* Move `redirect_to_root` from `lib/clearance/authorization.rb` to where
  it is used, in `app/controllers/clearance/sessions_controller.rb`, and
  better reveal its intent by re-naming it to `avoid_sign_in`.
* Re-set `config.secure_cookie` to its original value in an `after`
  block in a test to teardown and avoid leakage across tests.
* Use `_url` suffix in config name to match [RFC 2616 spec, section 14.
  14.30](http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.30),
  which states the Location response-header field should use an absolute
  URI for 3xx responses.
* The default value for `redirect_url` is the string path `'/'` as a
  previous compromise to make it less likely users will run into an
  issue if they don't want to define a root route. The string value avoids
  potentially confusing `NoMethodError: undefined method 'root_url'`
  errors.

https://github.com/thoughtbot/clearance/pull/281