[This introduces some new API functions.]

* Adds new flag to the connection context indicating SO_REUSEADDR
  should be set.
* Adds max number of retries constant for when connect() hits
  EADDRNOTAVAIL.
* Adds new function, redisAsyncConnectBindWithReuse(), letting
  clients enable this functionality.

[Removed trailing whitespace in new header lines.]

Closes #264