Date formatting depends on current locale

In `metadata.py` dates are formatted with `.strftime()` on date objects.  This method is locale-aware and formats dates according to the current environment locale, that is, the value of `$LANG` or `$LC_*`.

Hence, formatting of dates in posts depends on the environment in which Tinkerer is run, and **ignores** the `language` setting in `conf.py`.  

To build an English language blog on a system with, say, a German locale, you currently need to set `LANG` to `en_US.utf8` explicitly (or a similar English-language locale) to make the post dates appear in English.  Otherwise they will be German, i.e. `Januar 1, 2013` instead of `January 1, 2013`.

I can submit a pull request to fix this issue, but it would introduce a dependency against the Babel i18n library.  Is that acceptable?