In locales other than en_US, as a result of missing jQuery locale
files, the DatePicker defaulted to English whenever day and monthnames
were used, breaking validation. Needed to change official locale files
before adding, because Zend_Date and jQuery day/monthnames not matching
again breaks validation.
Removed hard setting the names to uppercase, breaking validation for
other locales
Changed order in convert_iso_to_jquery_format(), to prevent EEE(E)
settings from being overwritten
Added a check for existing locale files, and made DatePicker fallback
to ISO yyy-MM-dd if a missing locale file would otherwise break
validation.
Added documentation for the DateField
