all clients of python locale to nikola.utils (neutral refactor)

moved function get_month_name from nikola/plugins/task/archive.py to utils.py
moved function s_l from nikola.py to utils.py
renamed arg 'locale' -> 'lang' in get_month_name because is called with a selector of nicola translation, not a locale

nitpick: archive.py used no __future__ unicode_literals__ , utils.py yes used it.
explicit conversion lang -> locale (neutral in linux)

In functions that need to pass a locale, we prepare the (locale_name, encoding) in pylocale

When a function receives a 'lang' ( a selector of nikola translation ) and pases to a callable that expects a locale, we were using an implicit conversion.

We change that to an explicit conversion locale_from_lang(lang) -> locale , but in the linux codepath still we convert by identity, so it is a neutral change for linux.
In windows theres some translation, but before it was totally broken; still broken but better.

extracted logic in utils.Functionary.current_lang dealing with locales to guess_lang_from_current_locale
all functions dealing directly with locale are win compatible (linux neutral)

get_month_name and s_l now with codepath supporting windows, linux neutral
guess_lang_from_current_locale upgraded similar to the others, linux neutral
trivial: deleting an unused import in archive.py to clean flake8

Now the world 'locale' doesn't appear in nosetests output (windows)
Doing init --demo + build : normal, no warnings about locale (windows)