Option to not use resolvers.txt

[HarryR]

Should be able to use a single static resolver, e.g. localhost

Additionally, if the resolvers file contains less than a specific number it throws errors:

Traceback (most recent call last):
  File "dnsbrute/__init__.py", line 64, in run
    resp = self.bruter.query(dnsname, query_type)
  File "dnsbrute/__init__.py", line 238, in query
    resolver.nameservers = random.sample(self.resolvers, sample_size)
  File "/usr/lib/python2.7/random.py", line 323, in sample
    raise ValueError("sample larger than population")
ValueError: sample larger than population