[libsocialcache] Initial work on Facebook picture cache

This commit provides the first work on libsocialcache.
libsocialcache is a library that is used to save and
load data that comes from social networks, and are
cached locally.

libsocialcache includes a library that provides classes
that wraps sqlite databases. While org.nemomobile.socialcache
QML import provides higher level models that can be used
to access the cache.

Right now, only pictures cache from Facebook is
implemented, and Qt4 implementation is not working.
Added support for process semaphore
Added support of a singleton downloader

This commit provides Facebook image downloading as a singleton
instead of embedding it into each model, leading to less
concurrent write access to the db and collisions between
downloading tasks.
Added fixes

This commit adds some fixes, like the ability to remove photos
automatically, and a bit of documentation. This commit also
changes the name of DatabaseManipulationInterface class to
AbstractSocialCacheModel, that is better.
Merge pull request #1 from SfietKonstantin/master

[libsocialcache] Initial work on Facebook picture cache
[libsocialcache] Added packaging files
Merge pull request #3 from SfietKonstantin/master

[libsocialcache] Added packaging files
[bugfix] spec file bugfixes
Merge pull request #4 from SfietKonstantin/master

[bugfix] spec file bugfixes
[bugfix] Fixes build
Merge pull request #5 from SfietKonstantin/master

[bugfix] Fixes build
[libsocialcache] Basic work on feeds support

This commit includes some basic work for feeds for Facebook
and Twitter. It provides the capability to store them in a database,
as well as the capability to fetch them all.
More work on models

This commit includes more work on models, especially on
Twitter posts model. It also includes fixes, like in
picture database, where images are sorted DESC only
in "All" albums.

This commit also includes an optional SyncHelper component
(that can be disabled for desktop builds by passing nodeps
to qmake). This component can be used to track if a social
sync is going on, and adapt the display if needed.
Added KeyProviderHelper

The KeyProviderHelper is used to get keys from
social networks via sailfishkeyprovider, directly
from QML.
Added support of rollbacks
Merge pull request #2 from SfietKonstantin/devel

Facebook and Twitter posts support