Rework Buffer's 'contents-modified' event
Rework Buffer's 'contents-modified' event

This event now fires whenever the content of the buffer changes (after 
a rate-limiting delay) with a single boolean indicating the modified
status of the buffer. There's now a separate event called
'modified-status-changed' to indicate events that change the boolean
value of the isModified method, so we don't need to fire
'contents-modified' when the underlying file is deleted for instance.

pombredanne/status-bar,devoncarew/status-bar,mertkahyaoglu/status-bar,ali/status-bar,TShapinsky/status-bar,Abdillah/status-bar