Added Slovak language localization.

Based on the en_GB translation. Copied Plural-Forms from pl since it's
similar, not sure if identical, though.

Note that date format at uistr.py:42 seemed to be outdated (didn't
translate correctly), so I used the format from uistr.py source, which
makes the translation work. Also, some of the line numbers seem to be
outdated.
Merge pull request #57 from kiith-sa/locale-sk

Added Slovak language localization.