Change == to is_password?

This is more a discussion starter then a complete pull request.
I have found that the use of == confuses a lot of people who are making the switch to BCrypt, thinking that somehow their password is decrypted to a plaintext string which is then compared to the posted password.
Also overriding == is bad practice IMHO, because it gives unexpected behaviour here.
I've changed the read me to reflect this, which makes things a bit clearer for the first time user.  If you think this is a good idea, then it would be smart to change the actually implementation as well.