Add DNS injection test

This test is able to detect censorship when the answers to UDP based DNS queries are being spoofed.

It works even if you are outside of the country where you wish to perform the measurement, you just need an IP address inside of the country.

Note: this test was suggested by zion on trac here: https://trac.torproject.org/projects/tor/wiki/doc/OONI/Tests/DNSInjection

This will work for example to test which sites are being filtered inside of china:

`./bin/ooniprobe nettests/experimental/dns_injection.py -f inputs/ooni-inputs/processed/alexa-top-1k.txt -r 220.192.0.1`

    ###########################################
    # OONI Probe Report for DNS Injection test
    # Sat Mar  2 20:28:26 2013
    ###########################################
    ---
    options:
      collector: httpo://nkvphnp3p6agi5qq.onion
      help: 0
      logfile: null
      no-default-reporter: 0
      parallelism: '10'
      pcapfile: null
      reportfile: null
      resume: 0
      subargs: [-f, inputs/ooni-inputs/processed/alexa-top-1k.txt, -r, 220.192.0.1]
      test: nettests/experimental/dns_injection.py
    probe_asn: null
    probe_cc: null
    probe_ip: null
    software_name: ooniprobe
    software_version: 0.0.10
    start_time: 1362248906.0
    test_name: DNS Injection
    test_version: '0.1'
    ...
    ---
    injected: true
    input: facebook.com
    queries:
    - query: '[Query(''facebook.com'', 1, 1)]'
      query_type: A
      resolver: [220.192.0.1, 53]
    test_name: test_injection
    test_runtime: 0.3477039337158203
    test_started: 1362252506.732914
    ...
    ---
    injected: true
    input: youtube.com
    queries:
    - addrs: [78.16.49.15]
      answers:
      - [<RR name=youtube.com type=A class=IN ttl=31293s auth=True>, <A address=78.16.49.15
          ttl=31293>]
      query: '[Query(''youtube.com'', 1, 1)]'
      query_type: A
      resolver: [220.192.0.1, 53]
    test_name: test_injection
    test_runtime: 0.3509399890899658
    test_started: 1362252506.733631
    ...
    ---
    injected: true
    input: twitter.com
    queries:
    - addrs: [59.24.3.173]
      answers:
      - [<RR name=twitter.com type=A class=IN ttl=21937s auth=False>, <A address=59.24.3.173
          ttl=21937>]
      query: '[Query(''twitter.com'', 1, 1)]'
      query_type: A
      resolver: [220.192.0.1, 53]
    test_name: test_injection
    test_runtime: 0.351452112197876
    test_started: 1362252506.737242
    ...
    ---
    injected: false
    input: google.com
    queries:
    - failure: deferred_timeout_error
      query: '[Query(''google.com'', 1, 1)]'
      query_type: A
      resolver: [220.192.0.1, 53]
    test_name: test_injection
    test_runtime: 3.0029098987579346
    test_started: 1362252506.73108
    ...
    ---
    injected: false
    input: yahoo.com
    queries:
    - failure: deferred_timeout_error
      query: '[Query(''yahoo.com'', 1, 1)]'
      query_type: A
      resolver: [220.192.0.1, 53]
    test_name: test_injection
    test_runtime: 3.003868818283081
    test_started: 1362252506.734332
    ...