reverse hijack functionality

.login_user saves the id of the hijacker
.reverseHijack reverses the last hijack with the id saved in the session
named url "reverseHijack" calls .reverseHijack:
 -> reverses the last hijack(redirect to login_redirect_url or reverse_hijack_login_redirect_url) or 404 if no hijacker
added reverseHijack link and close button to notifications template
update readme
Merge pull request #15 from yannik-ammann/master

reverse hijack functionality