Implement FixedZone function

[nathanielc]

From the SPEC:

FixedZone creates a location based on a fixed time offset from UTC.

FixedZone has the following parameters:

• offset duration Offset is the offset from UTC for the time zone. Offset must be less than 24h. Defaults to 0, which produces the UTC location.

Examples:

fixedZone(offset:-5h) // time zone 5 hours west of UTC
fixedZone(offset:4h30m) // time zone 4 and a half hours east of UTC

[russorat]

dupe of #406 ?