[chore]make login and sign-up same page
[feature]user can login via FB
[feature]add image_url to users to record url-format cover-image
[feature]user can login in via Google oauth2
[feature]make user's email and provider together uniq
Merge pull request #92 from peterzd/omniauth

Omniauth
Merge pull request #93 from peterzd/dev

Dev