Yunus/issue improvements

- Subscribed_emails list is implemented for issues.
- Email commit author as well

@RISCfuture @RickCSong 
If you only use inclusion, it doesn't work well. For ex: 

validates :priority, inclusion: {in: (0..3).to_a}

i = Issue.last
i.status = 2.3
i.valid?          => returns true
i.priority        => returns 2