Move existing unit test into spec/units

To make room for acceptance tests in spec/features.
Install Capybara

We don't really need it yet, but it provides a nice DSL for writing
acceptance tests with RSpec, and we'll probably want actual Capybara
functionality later anyway.
Add failing acceptance tests for redirected and archived URLs
WIP: Implement Site.create
WIP: Implement Site#create_mapping
Introduce Mapping class
WIP: Implement Mapping.create
Implement Mapping constructor