Preparsed offset string issues

Fixes #152. Everywhere I can see tzOffset being used, it is expected to a be a number. But node-preparse ensures that it will be a string. e.g.,

```
    "Asia/Seoul": [
      [
        "-507.8666666666667",
        "-",
        "LMT",
        "-2493072000000"
      ],
      [
        "-510",
        "-",
        "KST",
        "-2053900800000"
      ],
      [
        "-540",
        "-",
        "KST",
        "-1293926400000"
      ],
      [
        "-510",
        "-",
        "KST",
        "-1167696000000"
      ],
      [
        "-540",
        "-",
        "KST",
        "-498096000000"
      ],
      [
        "-480",
        "ROK",
        "K%sT",
        "-264902400000"
      ],
      [
        "-510",
        "-",
        "KST",
        "-39484800000"
      ],
      [
        "-540",
        "ROK",
        "K%sT",
        null
      ]
    ],
```

I would add a test case, but jake dies on me when I try to test it. Use preparsed data and setTimezone will cause the error.