Makes any DbxException originated from DropboxDataManager handled by the class who is using it
Removes unused imports from MemoriesAdapter
Encapsulates the DbxDatastore opening procedure
Removes the save memory logic from MemoryViewerFragment
Encapsulates the close datastore procedure into closeDatastore() method
Refactors MemoryModel

Now there's no reference to a DbxRecord, completelly transfering the responsability to DropboxDataManager.
Removes unused imports from MemoryViewerFragment