uniqueItemsProperties - combined unique properties

[atng]

PR in response to the feature request in issue 107.

This PR impliments the suggestion by @epoberezkin by changing the keyword implementation of uniqueItemsProperties.

It extends the uniqueItemsProperties to accepts arrays of strings in addition to strings as items in the array.

You can extend this keyword to accept arrays of strings in addition to strings as items in the array to pass to the keyword.

e.g.

With the schema:

{"uniqueItemProperties": [["email", "username"]]}

[{"email": "one@email.com", username: "one"}, {"email": "two@email.com", username: "two"}]

Would pass.

[{"email": "one@email.com", username: "one"}, {"email": "one@email.com", username: "two"}]

Would pass.

[{"email": "one@email.com", username: "one"}, {"email": "two@email.com", username: "one"}]

Would also pass.

[{"email": "one@email.com", username: "one"}, {"email": "one@email.com", username: "one"}]

But one one would fail since both the username and email matches..

[atng]

@epoberezkin