refactoring & stuff:

- refactor code to split storage and retrieval into 2 classes each
- add base method because code is the same for retrieval
- fix a little the exception behavior
- javadocs coming
- instagram favorites are in fact picture objects -> removed the
avors and reuse the picture avro. store them in a separate family,
just like the ordinary pictures.
- uniform API to add items via JSON string(s)
- to discuss Monday about the methods: storeUser(json, internalID)