Updated version number to 1.7.3
Fixed a few bugs

- fixed a bug where having in the "disabled_dates" property a rule like
['* * * *'] (to disable all dates), and then, in the "enabled_dates"
property having rules that enable only dates in the future, would send
the script in an infinite loop; thanks Miguel Avila;
- fixed a bug where if a default value did not have days, months and
years but only fragments (like, "Feb" or "February" when format was set
to "F" or "M" respectively) the current date was used as default date
instead; thanks to MSP;
- fixed a bug where, when searching for the first selectable date the
library would sometimes fail and incorrectly show the month *after* the
first selectable month;
- fixed a bug that was introduced in the previous version that could
break the script if ranges were used in the "disabled_dates" or
"enabled_dates" properties;
- fixed an issue where if the only the "months" view was visible, the
user could still use the "next" and "previous" buttons to change years;
thanks to MSP;