Ensure default timezone offsets get cast as a Number

The pre-parsed json data has offsets as a string.  Unfortunately
timezone-js was not casting them to a number so in many cases the new
calculated times were wrong (e.g. 5 + "60" = "560" instead of 65) or
caused NaN errors when passed to date.setUTCMinutes (e.g. 5+"-60" = "5-60").
Merge pull request #160 from sdemjanenko/cast_offset

Ensure default timezone offsets get cast as a Number