Support of Persistent Connections

[lfcipriani]

As specified in [1].

Should also support configuration of keep-alive sessions.

[1] http://www.w3.org/Protocols/rfc2616/rfc2616-sec8.html#sec8.1
[2] http://en.wikipedia.org/wiki/HTTP_persistent_connection

[rogerleite]

To start this, i have to fix this issue savonrb/httpi#41 from httpi.