Add failing test for parsing top-level port in IPv4 records

[cdzombak]

This PR adds a failing test exposing the issue @dadrian and I tried to work around in #9.

The schema in the test_parses_ipv4_records test case is loosely adapted from the full ipv4_host + ztag_ssh_banner schema from zmap/ztag#68. The fixture JSON is a stripped-down version of the modified ZTag output @dadrian and I used to test zmap/ztag#68 (see https://gist.github.com/cdzombak/aba9071ed41ebb4a6fbba36259457d30).

Running tests (python setup.py test or nosetests .) gives the following failure:

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ERROR: test_parses_ipv4_records (zschema.tests.CompileAndValidationTests)
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Traceback (most recent call last):
  File "/Users/cdzombak/code/zmap/zschema/zschema/tests.py", line 323, in test_parses_ipv4_records
    ipv4_host_ssh.validate(json_fixture('ipv4-ssh-record'))
  File "/Users/cdzombak/code/zmap/zschema/zschema/compounds.py", line 194, in validate
    if subkey not in self.definition:
  File "/Users/cdzombak/code/zmap/zschema/zschema/keys.py", line 19, in __eq__
    return self.port.__eq__(other.port)
AttributeError: 'unicode' object has no attribute 'port'