Offline mode support, part 1

Refactor MojangAccount so it exposes a less generic interface and supports login. Hide the ugly details.
Yggdrasil tasks are now only used from MojangAccount.

logic/auth/flows/AuthenticateTask.h

@@ -30,7 +30,7 @@ class AuthenticateTask : public YggdrasilTask
 {
 	Q_OBJECT
 public:
-	AuthenticateTask(MojangAccountPtr account, const QString &password, QObject *parent = 0);
+	AuthenticateTask(MojangAccount *account, const QString &password, QObject *parent = 0);
 
 protected:
 	virtual QJsonObject getRequestContent() const;