Trapping all redis exceptions rather than just ConnectionException - this
will pick up the OOM errors as well.

Adding an exponentially increasing backoff timeout for reconnecting to the
redis server.

Log output now:
> tangerine:~/work/jw/logstash-forwarder$ PYTHONPATH=`pwd` ./logfile-forwarder ./test.conf
> 2014-10-31 17:04:57,745 INFO     output Redis: connecting to localhost:6379
> 2014-10-31 17:05:00,587 ERROR    output Redis: OOM command not allowed when used memory > 'maxmemory'.
> 2014-10-31 17:05:10,595 INFO     output Redis: connecting to localhost:6379
> 2014-10-31 17:05:10,596 INFO     output Redis: reconnected to server localhost:6379

Note lack of fatal exception, and new reconnect.

[Delivers #81770106]