Overwrite pystun server list.

* Non-responding servers in pystun's builtin list are causing node to
  crash upon starting up.
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.
updated STUN server list.
Merge pull request #2 from gubatron/fix_STUN_check

network_utils.stun_servers_status_report(server_list)
Various build and style fixes.

* Rename list to _STUN_SERVERS.
* Remove stun.iptel.org
* Abstract get_NAT_status to work with a specific hostname.
* Remove summary report to enable better automation.
* Add a main function to call test_stun_servers.
Merge pull request #922 from Renelvon/fix_STUN_check

Overwrite pystun server list.