Extract AC::Caching::Sweeper to a separate file
Load AC::Chaching::Sweeper if AC is defined when loading AR Observer

Without this, AC::Cashing::Sweeper could never be loaded if Rails initializer
loads AC before loading AR, which actually is Rails' default behaviour.
Merge pull request #24 from amatsuda/ac_sweeper_not_found

AC::Caching::Sweeper is not properly loaded