api/auth: Adding key in user struct inside addKeyToUserInDatabase function

This makes more sense than adding the key in the struct before the
function call. Also passing only the new key to gandalf api.
api/actions: created atomic actions to use in key addition into user

Refers to #300
api/auth: creating and using pipeline of actions in addKeyTouser function

Closes #300
api/actions: added comment to addKeyInDatabaseAction
Merge branch 'master' of github.com:globocom/tsuru