Use recommended gem structure

RubyGems and Bundler know how to handle gems that extend other gems
as long as their naming convention is followed. If the name of the
gem is active_remote-cached, they will attempt to load the gem with:

    require 'active_remote/cached'

Reorganize the gem structure to follow the convention. Leave
active_remote-cached.rb in case someone is using an older version of
RubyGems or Bundler.
Use an in-memory cache store by default
Add a Railtie to wire things up in Rails
Update usage in the README

Update the README with info about configuration and Rails as well as the
local cache option overrides.
Use Github-documented language hinting in examples

The official Github documentation for specifying a language for syntax
highlighting in code examples is "ruby", not "Ruby". Fix it.
Merge pull request #3 from liveh2o/railtie

Default cache provider and Railtie