updated README with explanation of new per-exception class retry delays
updated README with explanation of new per-exception class retry delays

thenovices/resque-retry,lantins/resque-retry,lantins/resque-retry,jzaleski/resque-retry,lantins/resque-retry,jzaleski/resque-retry,seomoz/resque-retry,seomoz/resque-retry,thenovices/resque-retry,thenovices/resque-retry,seomoz/resque-retry,jzaleski/resque-retry