Added: 'object:removed' event

Inlined removeFromArray call (because it didn't return whether indexOf returned -1 or not so I couldn't know if 'object:removed' had to be fired or not
Added: 'canvas:cleared' event

When the canvas is cleared, it doesn't fire any 'object:removed' event, on purpose
Merge pull request #388 from FremyCompany/patch-1

Add 'object:removed' and 'canvas:cleared' events