[feature]create invitation model
[chore]change name from invited_user to receiver
[feature]methods for user to show received invitations
[feature]create invitations_controller
[feature]user can invite others for the card
[feature]add `accept` and `reject` methods to invitation model
[chore]fix bug on album show page about the @user
[chore]add link to invitation page for card show page
[chore]fix bug that choose label click all input
Merge pull request #43 from peterzd/invitation

Invitation
Merge pull request #45 from peterzd/dev

Dev