added timeout to startDigging and err argument to event

 * sends out event with err=true if block is whole after 5 sec
 * added optional callback parameter to startDigging()
 * updated API docs
 * updated exampes/digger.js to warn about spawn protection zone
Merge branch 'diggingTimeout' of github.com:vogonistic/mineflayer

Conflicts:
	lib/plugins/digging.js