User authentication

Add ability for users to log in and log out. Resolves #10 
Something @pk-nb and I worked on was to create a separate `Session` class with its own validations that could be used with `form_for`.  Something like:

```ruby
class Session
  include ActiveModel::Model

  attr_accessor :login, :password

  validate :login, :password, presence: true
  validate :user_account_exists

  private

  def user_account_exists
    # check for user and use User.authenticate
  end

end
```