Establish persistance connection with keep-alive

[lbdremy]

As it is pointed out in the RFC of HTTP (http://www.w3.org/Protocols/rfc2616/rfc2616-sec8.html), a persistance connection has several advantages when a client makes multiple requests on the same server in a short amount of time, which would be good to benefit since our case.

[lbdremy]

By default http requests are performed with the http.globalAgent which by default set Connection: keep-alive. (http://nodejs.org/api/http.html#http_class_http_agent)