[chore]move import into rake task
[feature]introduce notification model
[chore]move joins_album method into album model
[feature]send_notification after user joins a card
[feature]notifications for invitations
[feature]post photo will send notifications to followers
[feature]notifications when user comments on photo
[feature]sends notification after user likes photo
[feature]tagged user will get notification
[feature]user can accept or reject an invitation in alerts page
[feature]user can list his notifications in the alerts page
[feature]effect for accept/reject invitations
[feature]add is_read to notification, add effect for notifications
Merge pull request #95 from peterzd/alerts

Alerts
[chore]add count to dropdown's Alerts
Merge pull request #96 from peterzd/alerts

[chore]add count to dropdown's Alerts
Merge pull request #97 from peterzd/dev

Dev