Be more strict about the `secure` flag on the old_user cookie
Make sure the old_user cookie gets removed when it's empty
Pretty icon next to the switch back links
Docblock update
Add an extra auth check before the nonce verification
Security hardening for sites that use HTTPS in the admin area and HTTP on the front end.

This is quite a substantial code change, but all it does is change the old user cookie structure to one which adheres to WordPress' own structure: an old user cookie which is alway sent but not used for authentication (akin to the logged_in cookie); an old user auth cookie for HTTP connections (akin to the auth cookie); and a secure old user auth cookie for HTTPS connections (akin to the secure_auth cookie).

This prevents an auth-level cookie from being sent over HTTP when the admin area is served over HTTPS and the front end is served over HTTP.
Docblock updates
Move our `function_exists()` logic above the corresponding docblocks
Rename the `secure_logged_in_cookie()` method to `secure_olduser_cookie()` for clarity
Bit of DRY on a rainy day
Readme updates
Unit tests for 'Tested up to' and 'Stable tag'. Inspired by Marko Heijnen's tests in his Tabify Edit Screen plugin.
Update PHP version in Travis config
1.0