i18n_subsites: consider templates lang, expand docs

this commit removes the need to make a dummy translation for
the language in which the templates are written.
This only affects themes using jinja2.ext.i18n.
The I18N_THEMES_LANG is introduced to address this issue.

Also expanded the docs for making gettext .po files with babel.
i18n_subsites: fix HIDE_UNTRANSLATED_CONTENT

1. was removing items of an active iterator -> only first worked
2. Article inherits Page, so first check if is Article
i18n_subsites: improve develop server support

1. main site url root fix
2. support autoreload mode
Also removed trailing slash on dir name.
18n_subsites: use configure_settings() to sanitize

Also needed for LOCALE overrides, etc.
Merge pull request #145 from smartass101/master

i18n_subsites: consider templates lang, expand docs, several fixes