Adding check for when no tags exists

the_tags is doing this automatically, but I forgot it when I was creating
my own implementation, hence triggering a warning when the code reached
the foreach loop.
Correcting translation package name
Structuring headings for SEO

On the home page (and list pages, for now) the blog title is the H1
heading, marking the title of the page. That's also where we want links to
articles, being their titles, but these titles are not he main H1 in the
page.

On the single page, however, the post's title becomes the page title and
doesn't need to be a link anymore.
Adding AddThis sharing buttons

For now it's hard-coded. Tried using the AddThis plugin for WordPress, but
adding custom widgets with it is totally broken when you want to have a
user-configurable list of services or style. Will try to add this to the
theme's configuration later.
Adding a proper theme setup function