Make current_user method return nil when user not found
properly memoize current_user, even if nil
update changelog
fix datamapper specs for password_confirmation
Merge pull request #516 from arnvald/fix-datamapper-tests

fix datamapper specs for password_confirmation
Make login_from session return nil instead of false

This method is not in public API, it's protected, but should
behave consistently anyway.
update supported ORMs and providers in readme
Merge pull request #509 from arnvald/current-user-returns-nil

Make current_user method return nil when user not found
Fix wrong mongomapper save! behaviour

Mongomapper's save! method was overriden to call save, which resulted
in errors not being raised when object was invalid.
Mongomapper supports save! method, however it accepts only :safe option,
it does not work with validate: false, so this option needs to be removed.

Fixes issue #151
Better behaviour for non-validate save! in MongoMapper

My previous commit changed the way Sorcery behaves. I've thought it's
better to keep the previous behaviour, but do not override `save!` method
Use find_by_id instead of find when looking for user