Fix for #701: adds a 'strftime' jinja fiter that uses LOCALE

This PR adds a new Jinja filter `strftime` that will solve the issue #701. Built-in `strftime` wasn't using the locale set by Pelican while rendering (probably because of some sandboxing done by Jinja).

Usage:

    # settings
    LOCALE = "pt_BR.utf8"

    # template
    {{ article.date.strftime('%B de %Y') }}
    {{ article.date|strftime('%B de %Y') }}

    # output
    December de 2012
    dezembro de 2012