If header 'X-Recent-History-Store' set, dont store

Using the header `X-Recent-History-Store` users of the recent history
exchange can specify whether a message should be stored or not.
If set to false, the message with this header will not be stored.
Update README.md
Update rabbit_exchange_type_recent_history.erl
Merge pull request #6 from mweibel/dont-store-if-header-set

If header 'X-Recent-History-Store' set, don't store the message