Fallback to loading /usr/share/toxic/DHTservers.

First try ~/.config/tox/DHTservers, and fallback to the /usr/share
one if the former fails.

This should allow people just starting toxic for the first time to
connect to the DHT.
Merge pull request #80 from viric/dhtservers

Fallback to loading /usr/share/toxic/DHTservers.