Session allows to reuse SessionRequests to the same host by making them
keeping alive connection. Turns out, though, that libcurl already does
that for us, and we didn't gain any speedup we thought we'd get.
Usage:
```
Networking::Session s;
Networking::SessionRequest *request = s.get(url);
request->startAndWait();
warning("HTTP GET: %s", request->text());
s.close();
```
You can still use SessionRequest without Session (but you can't put them
on stack!):
```
Networking::SessionRequest *request = new
Networking::SessionRequest(url);
request->startAndWait();
warning("HTTP GET: %s", request->text());
request->close();
```
