Potential fix for client crash when node returns bad data

It's possible that a riak node can return a poorly formated response in a
PING, in this case, the client was crashing because resp was not nil
but also wasnt something that could be stringified.
Merge branch 'master' of git://github.com/mrb/riakpbc
Attempted fix for #54

When a node returns an error in ReqResp _before_ actually pulling the
response off the wire, close the tcp connection so that subsequent
requests dont try pulling the earlier response. Additionally, this
requires a change to try to re-dialing in the begining of ReqResp if
the node is not already connected
Merge pull request #55 from paperlesspost/fix_background_ping

Potential fix for background ping #54

After a couple of days in production, this seemed to do the trick