Fix bug with getlocale. Fix #690.

See https://github.com/getpelican/pelican/issues/690#issuecomment-16360522
From the `getlocale` doc :

    category may be one of the LC_* values except LC_ALL. It defaults to LC_CTYPE.

So `getlocale(locale.LC_ALL)` should not be used, This commit replace it with
`LC_TIME` as we are here concerned with date & time.