Add users using `devise` gem.
Add google OAuth2
Add twitter oauth authorization
login with `login` user property
Remove current password for update.

Perhaps It would be nice to remove change password for users thar login
with oauth.
Each user has a plan.
Add avatar uploading to user.
Merge pull request #1 from alemata/add-devise-for-users

[WIP] Add users using `devise` gem.