Added upsert method to move a specific item to the start.

[PrimozPernat]

Created a solution as mentioned in #201

[PrimozPernat]

@stfalcon @hbmartin @fre5h @philippeauriach Hello everyone. Can I bother one who has the write permission to go over the small PR and merge it? That would be appreciated.

[bevzaanton]

Hi @PrimozPernat ! Sorry that I did not reply for a while. PR looks very good. But why you made some fields and methods public? Maybe will be better if you make them protected. In this way users will have access to those fields and method by extending MessagesListAdapter.
And if you have time, can you add new upsert method to the documentation?

[PrimozPernat]

@bevzaanton Thank you for the replay. I changed the access modifier for the items list and the method generateDateHeaders to protected. But the inner class Wrapper<DATA> has to be public because if someone wants to extend the MessagesListAdapter he can't access item in the list.

P.S.: I will update the documentation at the weekend.

[bevzaanton]

@PrimozPernat thanks for PR! I will upload ChatKit version 0.3.3 with your changes.