Add new module ns_checker.py

Added a new module ns_checker :
    This module launch a simple query on each nameservers for the specified domain.
    Nameservers are dynamically retrieved. The FQDN is the only one mandatory parameter.
    It's also possible to add additional nameservers by appending them in nameservers list.

    The default resolver can be overwritten with my_resolver.nameservers parameter.