Improve resolv.conf handling

[negz]

Thanks for this great little library! As a heavy user of SRV records it's nice to not have to reinvent the wheel.

This PR addresses https://github.com/benschw/srv-lb/issues/2, and thus slightly alters the API:

• DefaultConfig() now returns (*Config, error)
• NewDefaultLookupLib() now returns (Lookup, error).

It also adds a few niceties around resolv.conf, namely:

• The lookup libs will now lookup records against all nameservers in a parsed resolv.conf sequentially. The first successful result will be returned. If all nameservers fail the last error will be returned.
• Wraps the call to dns.ClientConfigFromFile() in an interface so tests don't have to parse a real resolv.conf

[benschw]

thanks for the additions!

[negz]

Thanks for the merge. :)