Added next_letter to Driver

Added next_letter function to Driver in order to also update the
self.current_letter field.
Added next_page method to Driver

Added method to get the next page tag from the Driver class. Fixed tests
for next_page and next_letter to make sure they're updating the
current_page and current_letter attributes as well.
Changed use of Tag in Driver to Url

Created a new Url model class for the interface between Scraper and
Driver. Using BeautifulSoup Tags was coupled just a little too much. And
I ran into trouble trying to handle the Driver's crawl loop where I
needed to instantiate a Tag.
Add crawl loop to Driver

Added Driver.crawl() to launch the crawling logic and process all the
pages. Haven't added anything to output the podcast urls yet.