easier overriding of Api superclass (+ Appraisal gem)

Rails and plugins can be super opinionated, but with this minimal refactor, will be easier to add to custom projects. Which might be relatively common.

As updated in the readme:

Common tweaks
If your api controllers do not subclass from ApplicationController, use this snippet in your initializer before calling Swagger::Docs::Config#register_apis(...).

class Swagger::Docs::Config
def self.base_api_controller; Api::ApiController end
end

Also added Appraisal gem, because I needed to test against rails 3 for my legacy app.