Add Freebase service

This service is, according to the main site, a "A community-curated
database of well-known people, places, and things", it allows searching
for and offering information about a wide range of topics, in a
well-structured and uniform manner.

The most low-level API is the MQL query interface, that is a JSON-based
language, queries consist of a data graph (according to their data schema)
with blank places, that will be filled in in the reply.

https://bugzilla.gnome.org/show_bug.cgi?id=726486
freebase: Add Topic query API

With this API, structured data can be obtained about any Freebase
ID, including localized text and references to images.

https://bugzilla.gnome.org/show_bug.cgi?id=726486
freebase: Add search API

This API enables searching for search terms, returning amongst other
info the Freebase IDs usable on the topic API.

https://bugzilla.gnome.org/show_bug.cgi?id=726486
freebase: Add helper function to retrieve images

This helper function can be used together with the topic API,
if a returned element is an image, this function can be used to
retrieve a GInputStream to the image at a requested maximum size.

https://bugzilla.gnome.org/show_bug.cgi?id=726486
demos: Add freebase demo

This is a simple command line app that puts some of the api to work.

https://bugzilla.gnome.org/show_bug.cgi?id=726486
freebase: Add missing volatile hint on refcount counter

That is manipulated through atomic operators and should be marked as such.
Updated Polish translation
Release version 0.15.1
build: Increment version to 0.15.2