Allow to pass locales as atoms to MemorizedVocabulary

[dolfinus]

Hello.

I've already fixed passing locales as atoms into t! and t functions: #30
But MemorizedVocabulary module does not check locale before saving it to :ets. So you can easily add call locale(:en, ...), but then you cannot call t(:en, "path") because locale here is checked with normalize_locale function which accepts only strings.

What I've changed::

1. Added locale name normalization step to locale/2 function
2. Allowed to pass atom locales into normalize_locale/1 function
3. Added few tests for this case