Merge Domain model into User model

[ttimasdf]

Fix #4
If two users belongs to different domains are different, then why not take domain as a property of user?

By the way, there are still some issues on the sql object. So far, my patch seems to save all emails with same username with different domains under one user entry, although by get_or_create_user it should create a new one. the mail list page merely filter the mail from current domain, according to the mail id.