send host header with the GET request

[lrusak]

Changes in golang 1.6 don't allow http requests with a null host header.

This causes

HTTP/1.1 400 Bad Request
Content-Type: text/plain
Connection: close

400 Bad Request: missing required Host header
Traceback (most recent call last):
  File "./dockermon.py", line 146, in <module>
    watch(callback, args.socket_url)
  File "./dockermon.py", line 82, in watch
    raise DockermonError('bad HTTP status: %s %s' % (status, reason))
__main__.DockermonError: bad HTTP status: 400 Bad Request

To fix this we just need to include some host header. The fix was taken from here

see these links for more info.
golang/go#15276
golang/go#13624

[tebeka]

I prefer to use the real host name if it's a TCP connection and localhost if it's IPC.

[lrusak]

I've updated it with the requested changes.

[tebeka]

Thanks!