Refactor and test init_additional_STUN_servers.

* Make it accept any server sequence.
* Make it idempotent.
Test and refactor.

* Import modules, not names.
* Stricten port tests.
* Rename check_NAT_status to get_NAT_status.
* Test is_ipv6_address.
* Fix docstrings and test get_peer_url.
Merge pull request #841 from Renelvon/test_network_util

Mini refactor and test network util
If port arg is set then obey it don't ignore it
Missing CryptoPeerConnection method bug
Why check again? Redundant
Catch legacy message unpack exception
Merge pull request #873 from hoffmabc/decline-legacy-msgs

Hybrid network fixes
updated changelog for 2.1 and 2.2
Added 2.1 and 2.2 releases to changelog
STUN check fixes. Re Issue #816

 - Now STUN server is returning what should be the outer port on which
   it could listen to the OpenBazaar node (a random one)
 - The --server-port argument is ignored unless --disable-stun-check is passed.

The reason being default behavior is configured so that OpenBazaar will try to establish all its connections without user intervention.
A user that knows on what port they want OpenBazaar to run and who is manually forwarding ports should then read the help and understand the port is being ignored due to the default STUN check openbazaar attempts.

  Try setting a manual port, the one you have set on your router.
  `openbazaar --disable-stun-check --server-port <your_port> -d --log-level 10 start`

  Try letting openbazaar do its work by looking up a valid incoming server port via STUN, and to automatically do the UPnP port mapping on such port.
   `openbazaar start`
Merge pull request #845 from gubatron/stun-fix-attempt

STUN check fixes. Re Issue #816
Fix issue 861: configure.sh fails on mac os x
Merge pull request #878 from benemorius/fix_configure_script_for_mac

Fix issue 861: configure.sh fails on mac os x
Move exception out of crypto_util
Refactor guid exists check
Merge pull request #880 from hoffmabc/decline-legacy-msgs

Move exception out of crypto_util
Merge pull request #876 from gubatron/changelog-sync-issue-831

Changelog sync - Issue #831
Create new blocked_peers table
Add blocked peers to Settings page