Start outlining classes for file based reporting
Specify the type of FileReporter instance.
Define the copy and delete constructors for ReporterBase
Add copy contructor also to FileReporter
Make some attributes of ReporterBase public and change subclassing style
Integrate changes based on feedback from @bassosimone
Merge pull request #39 by @hellais
Add dns.hpp, the DNS resolver
dns.hpp: note that there is more work to be done
dns.hpp: allow to specify the nameserver
dns.hpp: add namespace
dns.hpp: check return value of resolve_ipv4/ipv6
dns.hpp: implement reverse DNS lookups
dns.hpp: make code easier to grok

Refactor the function processing the results to avoid the annoying
code organization that in some cases you return during the switch
and in some cases you fall through.
dns.hpp: document the code
dns.hpp: more documentation
dns.hpp: one attempt by default, allow to tweak that

Add an attempt string parameter, with default value "1", allowing
to control how many attempts EvdnsBase makes to resolve.

We use "1" by default becase OONI's tcpt does just one attempt
to resolve and then reports an error.
Add empty regress for net/dns.hpp