.buffer-newmessage => .buffer-newevent. newmessage is for plain/action

Rename the class that is added on a buffer event from `.buffer-newmessage` to `.buffer-newevent`.
If the buffer event is a Message.Type.Plain=1 or Action=4, the `.buffer-newmessage` class is also added.
Merge pull request #15 from Zren/unread

.buffer-newmessage => .buffer-newevent. newmessage is for msg.type 1&4