Add User model and specs

- tests for validation: first_name, last_name, email
- test for email uniqueness: there is a gem called shoulda-matchers
that could be used for something like this.  For now I just created
two users with the same email and verified the first was valid and
the second wasn't.
- last test was for a valid username between 2 and 32 characters
Merge branch 'development' of https://github.com/ericfransen/dinner_dash into development