network_utils.stun_servers_status_report(server_list)

Given a list of stun servers, probes them and prints out
a report. Useful whenever we want to update the list of
STUN servers to use.

```
>>> import network_util
>>> network_util.stun_servers_status_report()
probing stun.l.google.com...
:(
probing stun1.l.google.com...
:(
probing stun2.l.google.com...
:(
probing stun3.l.google.com...
:(
probing stun4.l.google.com...
:(
probing stun.ekiga.net...
:)
probing stun.ideasip.com...
:)
probing stun.iptel.org...
:)
probing stun.schlund.de...
:(
probing stunserver.org...
:(
probing stun.voiparound.com...
:)
probing stun.voipbuster.com...
:)
probing stun.voipstunt.com...
:)
probing stun.voxgratia.org...
:)
probing stun.xten.com...
:(

7 good servers
  stun.ekiga.net
  stun.ideasip.com
  stun.iptel.org
  stun.voiparound.com
  stun.voipbuster.com
  stun.voipstunt.com
  stun.voxgratia.org

8 bad servers
  stun.l.google.com
  stun1.l.google.com
  stun2.l.google.com
  stun3.l.google.com
  stun4.l.google.com
  stun.schlund.de
  stunserver.org
  stun.xten.com
>>>
```