Add Support for User Defined DNS Resolver Object

[stcz]

This MR allows passing a user defined resolver object (dns.resolver.Resolver()) to the DNS Requests. This is useful, when you use functions, and you want to pass special options to the resolver object.

The Settings timeout and nameservers are ignored, when a resolver is given.

Please review or comment. I just tested it in my use case, but tried to adapt for all use cases.

I'm currently unsure how to handle the test_dnssec() function, especially the dns.query.udp() for me, it is OK to ignore it.