Enables custom locale while feed generation.

Fixes wrong language of month names in feed links.

Related to #1198
Updates the custom_locale test feed files to match the correct month names
fix the parameters for a logging.warning call
Merge pull request #1499 from matrixise/fix_logger

Fix the parameters for a logging.warning call
Merge pull request #1399 from Scheirle/fix_feed_localized_url

Enables custom locale while feed generation.
advertise the i18n_subsites plugin as an addition to simple translations
note that {ARTICLE,PAGE}_PATHS may include files, add faq about using that
Add Multi-theme support

Adds multi-theme support using the new THEMES setting.
You can specify all the themes that you will be using in python
dicionary form. You can then inherit from the themes specified
in THEMES using the corresponding key in the dictionary.