Add support for differing locales

[punkstar]

There are some areas of the code that rely on Zend_Date::toString to convert Zend_Date into DateTime objects. This isn't a reliable way to do it as some locales do not generate strings that can be correctly parsed by DateTime::__construct.

We need to modify the code to be able to safely convert between Zend_Date and DateTime, regardless of locale.

Related Issues:

• Module fails when there are more than one shipping option #7