Add User model
Migrate users table
Add annotate gem
Annotate User model
Add test for User model
Ignore sqlite3-journal files
Validate presence of name in User
Validate presence of email in User
Test for email validity based on form
Test for case-insensitive uniqueness in email addresses -- with one caveat

The caveat: two users with identical email addresses can be created almost
simultaneously and still be considered valid, because at the time the second
one is validated, the first one hasn't been saved to the database yet.
Enforce email uniqueness through database

This takes care of the caveat mentioned in the last commit.
Convert emails to all-lower upon save
Add password_digest to User
Add password authentication