Add internationalization support.

- depend on Babel (for locale support)
- mark most obvious string to be translated (some with plural forms)
- add "locale" setting to user that sets `request._LOCALE_` (what locale/language to use)
- add support for syncing translation to transifex https://www.transifex.com/projects/p/substanced/resource/master/
- fix some issues that "extract_messages" was choking on while parsing xml
- change logout icon from 'pencil' to 'off'
- add "_" translationstring function to util module
- add initial translation for Slovenian (72%)
- add initial translation for German (1%)
- document internalization workflow into HACKING.txt
- add flake8 ignored error hints to setup.cfg according to pylons codestyle
add fr/de/it translations and update slovenian
make Babel as optional dependency as it doesn't support python 3.2
use babel (if installed) to display locale names instead of codes
Merge branch 'master' into i18n
coverage, dont depend on babel at runtime; instead use abbreviation of locale name