migrate to -[NSArray firstObject] for safety

Using -[NSArray firstObject] will return nil on an empty array while -[NSArray objectAtIndex:0] will throw an NSRangeException.
Merge branch 'firstobject' of github.com:pizthewiz/Cinder into pizthewiz-firstobject

Conflicts:
	src/cinder/app/AppImplCocoaBasic.mm
added some nil firstObject checks
Merge pull request #593 from cinder/objc_firstObject

Obj-c firstObject