Fix crash when finishing the wizard

As shown on this backtrace:

=13334== Invalid read of size 8
QCoreApplication::postEvent(QObject*,QEvent*, int)
Mirall::ownCloudInfo::slotReplyFinished()
[...]

==13334==  Address 0x1afb91d8 is 8 bytes inside a block of size 16 free'd
operator delete(void*) (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
QNetworkAccessManager::~QNetworkAccessManager()
Mirall::MirallAccessManager::~MirallAccessManager()
[...]
Mirall::ownCloudInfo::setNetworkAccessManager(QNetworkAccessManager*)
Mirall::ownCloudInfo::setCustomConfigHandle(QString const&)
Mirall::OwncloudSetupWizard::slotAssistantFinished(int)
[...]
Mirall::OwncloudWizard::basicSetupFinished(int)
Mirall::OwncloudWizard::slotCurrentPageChanged(int)
[...]
QWizard::currentIdChanged(int) (in
Mirall::OwncloudWizard::successfulStep()
Mirall::OwncloudSetupWizard::finalizeSetup(bool)
Mirall::OwncloudSetupWizard::slotAuthCheckReply(QString const&,
[...]
Mirall::ownCloudInfo::ownCloudDirExists(QString const&, QNetworkReply*)
Mirall::ownCloudInfo::slotReplyFinished()

The QNetworkManager was destroyed in a slot connected to a signal emited
by slotReplyFinished,  but we did not finish with the QNetworkReply yet.

Fix the crash by using QueuedConnection so the QNetworkAccessManager is
not destroyed before slotReplyFinished has completed.