Image URIs

This update adds the ability to scrape images using the "images" chainable method. It also does some general refactoring of the code such as some more variable renaming, minor modularization of the code for better readability, more tests, and moving the bulk of the logic into the craigslist.rb inside the lib/craigslist folder. It also privatizes a couple methods we don't want in the public API, and privatizes the modular methods that help with the method "last".

This update fixes #10.