dns detector: simplify code a bit by using AF_UNSPEC
dns detector: do not log tracebacks for expected exceptions

EAI_NODATA: no entry (A or AAAA) at all present in DNS
EAI_NONAME: not the requested kind of entry in DNS (but others)
dns detector: fix docstring
minor setup.py cleanups (use "with" to always close files, always load relative to BASEDIR)
dns detector: fix default value (fixes unit tests)
Merge pull request #31 from nsupdate-info/develop

some small fixes / cleanups for dns detector