Merge pull request #31 from nsupdate-info/develop

some small fixes / cleanups for dns detector
fix license story. fixes #32
simplified dns detector code. replaced options dictionary non-sense with
**kwargs
pin workaround for python bug more precisely
pep-0257
docstring
Add a new interface method af() to all detectors. This method returns
the address family that the specific instance of the detector is
configured to return.
automatically force DNS resolution to use the same address family than
the configured detector.
more webcheck urls
initial code for config file handling including "profiles" containing
presets of well known options
use a working default in unittest
initial proof-of-concept for config file support using built-in profiles
(preset shorthands) while retaining 100% freedom
tests and error handling
tests
small fixes, tests
use code from cli_helper
english
todos
pep8
workaround netifaces install issue