Add time zone support

ISO 8601 standard offset is also supported so the user can use an offset directly if they would like.  With this new feature, though, the user also has the option of specifying an alphanumeric time zone code.  A number of users would like to do this as opposed to calculating their own offsets.  Also, some time zones change their offsets based on the date (i.e. daylight savings time) and using a time zone code takes care of this case.  I have updated the readme to better explain this.