Make thread safety configurable

Had some free time and thought I'd give this a crack. Its very basic and should allow folks to use their preferred pooling gems. Should address: https://github.com/excon/excon/issues/330

The test is pretty rudimentary and I'm open to suggestions.
I think it might be good to make this a bit more explicit, maybe `:thread_safe_sockets`? I don't know that there will be a broader thread_safe option needed, but I try to protect the top level namespace when I can (and specificity is good in showing intention of options I think).