Release 2.0.0.

CHANGELOG:

- login method now doesn't accept the remember argument.
- new remember method for remembering logged in users.
- change in dependency: use Armor instead of PBKDF2 (same
  crypted_password still).
Clear the session on authenticate

I believe clearning the session before setting session authentication
is standard practice to avoid session fixation attacks.
Merge pull request #6 from bemurphy/thwart_fixation

Clear the session on authenticate
remember method doesn't take user anymore.

We were taking it because:

1) We forgot about it
2) We feel it may be for future-proofing if we wanted
   to do some complicated signing logic per-user.

After thinking through it, that signing logic might
best live in a different gem.
Merge branch 'master' of github.com:cyx/shield
Release 2.1.0: remember takes only an expire argument.