i18n improvements

*(Notes from [this commit](https://github.com/glueckpress/simone/commit/a035be9ca7ed0b4b8b2b88db2f8cfdbce16a796e):)*

Several suggestions for improved use of formatting functions, context and notices for translators. For reference see http://wceu2013.ottopress.com/ (once it’s back in place).

#### Key points:

1. **Deliver complete messages.** Splitting text strings in translatable substrings bears the premise those chunks could be used in other languages as they can in English. They often cannot.
2. **Use formatting functions mainly to keep HTML out of translatable strings.** Security and all. _s needs to work on that, too, in places.
3. **Use [Codestyling Localization](https://wordpress.org/plugins/codestyling-localization/) to verify translatability of your code and language files.** It supports context functions as well as translators notices and will notify you in case multiple textdomains can be found in your code (my-simone, twentyfourteen etc). Also nb.po does not seem to be a valid language file name. It probably should be nb_NO.po, accompanied by a
.mo file.

Neat theme otherwise! Will provide German translation once this pull request has been accepted.