Implement new, create, and show for short URLs

* Create migration and model for short_url
* Create views for new and show
* Create short_urls_controller
* Add helper methods to model for encoding and decoding slugs
* Add controller method to format the redirect url
Implment redirects
* Refactor all business logic into models

* Use figaro gem to track host, port, and path in ENV vars
* Move slug->URL logic into model
* Refactor model to expose two only two public methods
* Add before_save filter to ensure URL begins with http
* Add presence validation for URL