Reset contraints in prepareForReuse

When a cell with 0 messageBubbleTopLabelHeight is reusing a cell which had a non-0 height, we get "Unable to simultaneously satisfy constraints" exception.
Merge pull request #531 from walsh2000/develop

Reset contraints in prepareForReuse
add isMediaMessage to JSQMessageData. close #530.
revert changes from PR #531
animate new cells even when springiness is disabled. close #448.