Feature/custom cell identifier

In my project, I have to use different bubble images depending on the state of the message. But currently cell identifiers cannot be manipulated (or can it be?) and cell identifiers are set using a string combined of type, hasTimestamp, hasAvatar and hasSubtitle. Moreover, bubble image views are reused according to those cell identifiers. 

Thus I needed to somehow customize cell identifiers so I have added a function in the JSMessagesViewDelegate.