Add socket timeouts. fixes #26.

Add timeouts for socket operations.  ssbench-master now takes two new
arguments, --connect-timeout <float_seconds> and --network-timeout
<float_seconds>.  The first is a timeout for connecting to the Swift
proxy (or load-balancer or whatever), and the second timeout applies to
all socket operations after the connect.  They default to 10.0 and 20.0
seconds, respectively.

Also fix bug where the storage_url would get overridden with None.