Added the Auth0Service as a singleton through the classname

[JCombee]

Since the Auth0Service is registered under the name auth0 in the app container it doesn't load the instance when you use it in a constructor, since it is used in the authentication provider like this:

<?php

// ...

/**
 * Service that provides an Auth0\LaravelAuth0\Auth0User stored in the session. This User provider
 * should be used when you don't want to persist the entity.
 */
class Auth0UserProvider implements UserProvider
{
    protected $userRepository;
    protected $auth0;

    /**
     * Auth0UserProvider constructor.
     *
     * @param Auth0UserRepository       $userRepository
     * @param \Auth0\Login\Auth0Service $auth0
     */
    public function __construct(Auth0UserRepository $userRepository, Auth0Service $auth0)
    {
        $this->userRepository = $userRepository;
        $this->auth0 = $auth0;
    }

    // ...

}

#Hacktoberfest

Closes #103

[joshcanhelp]

Thank you @JCombee! I'll test this out first thing next week and merge this in.

[joshcanhelp]

@JCombee - I just realized that this is being based against the wrong branch. Can you switch to master?

Otherwise, this looks good and works on my end 👍