Bugfix: conversion of an empty string to a date or hour object

Bugfix: conversion of an empty string to a date or hour object
now makes the values of such an object 0 (null) instead of giving
a segmentation fault.
The class UTC combines the date and hour classes. The most basic
functions of the UTC class are now implemented.
These include constructors and conversion to and from String objects.
New functions: date::proper(), hour::proper() and UTC::proper().
Return true if the object holds a proper clock time and/or calendar
date; false if at least one value is out of range.