Add support pluralization support to Redis i18n

Changes behavior of I18n::Backend::Redis#lookup. Before it returned
value of "#{locale}.#{key}*" or nil (if the value was not set).

Now it returns value of "#{locale}.#{key}*", if it's missing it returns
hash of sub-keys or nil (if there are none).
Merge pull request #191 from ahenobarbi/master

Add pluralization support to Redis I18n