implements getRememberToken, SetRememberToken($value) and getRememberTokenName() methods to the Admin Model

after upgrading to larval 4.1.26, we must implements these 3 methods in
the
admin model.

http://laravel.com/docs/upgrade#upgrade-4.1.26
closes issue #28

add remember_token to admins table
make remember_token column in admins table nullable
Merge pull request #25 from Vinelab/fixe/issue-24

Fix Issue #24