renamed model getters and properties for method reconstruction
create getAvailableMetadata method #5

- implement constructor with metadata key prefix
- implement new methods in SlideShareModel class
- declare abstract getAvailableMetadata method
metadata creation of new imported posts #5

- fill created posts array in importer object
- use getAvailableMetadata() to retrieve object property and
reconstruct getter to reach the value
- renamed createAttachment() -> uploadAttachment()