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.
Revert "Disable sqlite same thread check. (Re:Issue #654)"

This reverts commit 03780eab62d3aabce9036e947bfa5faa77ece451.

* Disabled thread checks may cause sqlite3 to fail in unexpected,
  difficult-to-trace ways. The threading issue shall be resolved more
  systematically in the future.
Merge pull request #940 from Renelvon/enable_thread_checks

Re-enable DB threading checks
Ensure all messages have version tag
Add version tag to hellos
Update obelisk to unsystem rather than airbitz