Allow dynamic cache stores

ActiveSupport should allow dynamic cache store initialization just how database adapters work:

config.cache_store = :foo_store

If :foo_store is not recognized, AS should try to require the convention:

require 'active_support/cache/foo_store'

I'm working on a next gen memcached client, Dalli, and this would make Dalli a drop-in replacement for memcache-client but the user does need to require the store adapter currently:

require 'active_support/cache/dalli_store'
config.cache_store = :dalli_store

I will create a patch later this weekend.