Overwrite pystun server list.

### Motivation
Non-responding STUN servers are causing node to crash upon starting up.

### Changelog
* Manually override the server list used by `pystun`.

### Note
This is only the skeleton of the patch; the actual server list is not ready! Please follow the instructions in [this](https://github.com/OpenBazaar/OpenBazaar/issues/898#issuecomment-61666700) comment to help determine reliable STUN servers.

Fixes #898.

### Good STUN servers
* `stun.ekiga.net`
* `stun.ideasip.com`
* `stun.voipstunt.com`
* `stun.voiparound.com`
* `stun.voipbuster.com`
* `stun.voxgratia.org`

Stats submitted by: @gubatron, @SamPatt and me.