[nemo-qml-plugin-social] Bugfix to the Qt5 port

In Qt5, path provided to QUrl are more strictly checked.
In social, the paths were provided without the first "/",
resulting in a silent fail in the test program.

It is fixed in this commit. Therefore the test program
works now in Qt4 and Qt5.
Merge pull request #16 from SfietKonstantin/master

[nemo-qml-plugin-social] Bugfix to the Qt5 port