Implement configurable storage backend

[viralpraxis]

Resolves #234

Configuration example:

# config/initializers/rails_settings_cached.rb

RailsSettings.configure do
  self.cache_storage = ActiveSupport::Cache::RedisCacheStore.new(url: "redis://localhost:6379")
end

[huacnlee]

I'd like to accept these changes.

1. Please write a test for customize storage.
2. This storage must have a default storage.

[viralpraxis]

Tests are fixed (hopefully)

[viralpraxis]

@huacnlee seems like you used outdated configuration API version here: ed2ab02

Current implementation assumes this kind of usage:

RailsSettings.configure do
  self.cache_storage = ActiveSupport::Cache::RedisCacheStore.new(url: "redis://localhost:6379")
end