add textual diff for strings. fix #20

[andreineculau]

NOTE: a backwards-incompatible change, a failure now has expected, actual value, but also "details" (in this case, a text_diff detail)

An example of the new JSON output

      "errors": [
        {
          "reason": "not_equal",
          "key": "/body/args/whoarewe",
          "expected": "asdMikeJoe",
          "actual": "Mike_and_Joe",
          "text_diff": [
            {
              "del": "asd"
            },
            {
              "eq": "Mike"
            },
            {
              "ins": "_and_"
            },
            {
              "eq": "Joe"
            }
          ]
        }
      ]