http://atastypixel.com/blog/making-uitoolbar-and-uinavigationbars-background-totally-transparent/
- (void)drawLayer:(CALayer *)layer inContext:(CGContextRef)ctx
- (void)drawLayer:(CALayer *)layer inContext:(CGContextRef)ctx {
CALayer *imgLayer = [[CALayer alloc] init];
[imgLayer setContents:(id)[[UIImage imageNamed: @"toolbar.png"] CGImage]];
[imgLayer setBounds:CGRectMake(0.0f, 0.0f, self.frame.size.width, self.frame.size.height)];
[imgLayer setPosition:CGPointMake(self.bounds.size.width/2,self.bounds.size.height/2)];
[layer insertSublayer:imgLayer atIndex:0];
}