Added additional message; added sample timestamp (unix epoch)
Added datetime header to threadlist. If timestamp of current message is different than previous message, datetime header is shown (i.e., group messages by datetime). TODO: consider using some library like momentjs to prettify the datetime header as well as compare by day (rather than current version that compares by epoch second).
Added style for datetime group header (.thread-list .thread-item-groupheader). Also, added .threads-list .selected>.thread-item to correctly show selected item since ThreadItem is now moved down one level in components for DataList