Change default response format.

Before it was incompatible with how it would be called later, and would raise an
exception.
Allow the consumer to load rspec.

Requiring RSpec breaks if we're using rspec 2. It's best we just let the
consumer handle setting up the test environment.
Lazy load the billy proxy.

This should allow forking preloaders (zeus & co.) to work with it out of the
box, also stops us starting a server when the spec run in question does no even
involve capybara or JS testing.
Merge branch 'master' of https://github.com/cwninja/puffing-billy