[libsocialcache] Use IdlePriority for worker threads

To avoid starving GUI threads of processing, we should use the IDLE
priority for all worker threads.
Merge pull request #12 from chriadam/idle

[libsocialcache] Use IdlePriority for worker threads
[cleanups] Fixed project files

This includes removing dependency from tests to
libsocialcache since we are actually building
the tests statically.
Fixed license headers
Bump version
[cleanups] Uniformisation of SQL types

We simply use TEXT and INTEGER instead of VARCHAR(X)
to make the whole SQL tables more coherent. A Facebook
identifier will then always be stored as a TEXT,
same for urls and texts in general. Only numbers such
that counts and account identifiers are stored with
INTEGER.
API changes for Facebook images

Facebook images was using an old database
architecture, that provides useless fields.
These fields were also used by the whole API,
even if they don't have real usage.

This commit removes all these old fields,
including user images, and album cover images.
[feature] Added contacts support

Added support for synced contacts informations.
Most of the informations should go to the contact
database, but cached photos are stored in a
separated database.
[feature] Added downloader API

The downloader object that is used by FacebookImageDownloader
is now exposed as a generic object that can be used to download
from any source and update a database to set the cached file.
Bump version