Adds and tests new session endpoint

A user should be able to request a page that allows himself or herself to log
in. The server must simply return a form for doing so.

Issue: 6
Delete Play's default tests
Add user authentication to sessions controller

Users should be able to enter an email and password to log in. A route was added
to the sessions controller to allow credentials to be POSTed. If the login is
valid, the user is redirected and if not, the user must try again.
Add user authentication to User model

Emails and passwords are now correctly validated by the User model upon log in.
Redirects user after form validations errors

Previously, the form would render from the POST URL without redirecting. This
means refreshing the page would cause resubmission. Now, it has been changed to
a redirect and persists the form errors.
Tests authentication for SessionsTest controller

Added tests:
- _new_ route with flash error message
- _create_ route with invalid login
- _create_ route with valid login

Additionally, adds unique qualifier to User email in DB.

Issue: 6
Add tests for authentication in User model

Adds the following:
- Test for valid login
- Test for invalid email
- Test for invalid password

Issue: 6
Add annotation-based authentication
Add logout route

Adds ability to clear the current session for the user. Then, redirect user to
index page.

Issue: 6
Add tests for logout endpoint

Adds the following:
- Test to ensure redirect to index after logout
- _Fake_ test to ensure session is cleared

The test to ensure session is cleared does not actually test anything. I was
unable to find any way to ensure that the session is cleared using unit tests.
Perhaps there is something that can be done with 'functional' tests, using a
mocked browser.

Issue: 6
Merge branch 'issues/6'