Switch httpclient to use net module instead of sockets

This doesn't currently work with https connections. The old sockets module ignored return codes from SSL_shutdown, whereas net does not. This results in the following exception being thrown:

```nimrod
Traceback (most recent call last)
a9.nim(2)                a9
httpclient.nim(377)      getContent
httpclient.nim(358)      get
httpclient.nim(332)      request
net.nim(429)             close
net.nim(260)             socketError
net.nim(143)             raiseSSLError
Error: unhandled exception: No error reported. [SslError]
```
Does anybody no the reason for this?