Fixed 4 bugs:
    - Inconsistence in the get_random_url_safe_string between the
      initial and returned lengths.
    - Variable resolved_auth_ns undefined in check_0x20_to_auth_ns
      when a sample_size was given.
    - Condition unreacheable in compare_random_hostnames.
    - Double checking for isalpha in get_random_hostname and get_random_url_safe_string

It has also been improved the conformance to PEP8.
Merge branch 'bugs_captive_portal'

* bugs_captive_portal:
  Fixed 4 bugs:     - Inconsistence in the get_random_url_safe_string between the       initial and returned lengths.     - Variable resolved_auth_ns undefined in check_0x20_to_auth_ns       when a sample_size was given.     - Condition unreacheable in compare_random_hostnames.     - Double checking for isalpha in get_random_hostname and get_random_url_safe_string