Avoid passing locale-dependent string to dateutil

When a date or datetime object is passed to freeze_time, it is
converted to a string which is then later parsed with dateutil.

The conversion was done using the '%x' format character, which return
the appropriate date representation for the current locale. This means
that you get '01/02/13' for January 2nd with the C and en_US locales,
and '02/01/13' for the same date using most European locales. The
string is then parsed incorrectly by dateutils.

Using the isoformat method on date/datetime objects solves this by
returning a string in 'yyyy-mm-dd' format. This string is parsed
correctly by dateutil, regardless of the current locale.
Use SkipTest from nose for Python 2.6 compatibility

The standard unittest module only got SkipTest from Python 2.7.
Use Python 3 compatible except syntax
Try Danish, German, and French locales when testing locale handling

This just increases the chance that the user will have a usable locale
on his system.
Merge pull request #25 from mgeisler/master

Avoid passing locale-dependent string to dateutil