Add Gemnasium dependency status to README
Split Clearance::{Authentication,Authorization}

There has been confusion about the `authorize` method residing in the
`Authentication` module:

* The `authorize` method performs authorization - it denies access to
  unauthenticated users.
* It is assumed that controllers would override `authorize` for
  controllers that require specific authentication.
* It's sort of strange that `Clearance::Authentication` contains a bunch
  of authorization logic.

So, we:

* Split `Clearance::Controller` into `Clearance::Authentication` and
  `Clearance::Authorization`, both of which get mixed into
  `Clearance::Controller`.
* Mix `Clearance::Controller` into `ApplicationController` in the install
  generator.

Read more:

https://github.com/thoughtbot/clearance/pull/268
https://github.com/thoughtbot/clearance/issues/257
Add Ruby 2.0 to tests/travis

Ruby 2.0 requires Rails 3.2.13, currently in rc. Update appraisals to
run only that version of rails under 2.0. Update travis build matrix to
do the same.
Bump to 1.0.0.rc6
Clean up NEWS.md

* Replace inconsistent name credit with link to full set of contributors
  so some people aren't over-thanked and others aren't under-thanked.
* Add dates for all releases.
* Use more Markdown so it looks better on GitHub.
* Wrap at 80 characters.