Custom icon

This pull request adds the ability to create a custom SCLAlertView, using the same title, subtitle, buttons, etc. as the normal alert view, but also being able to customize the icon image/color and button colors.

As you can see from the provided example, using the below code has the following pretty nice result:

![demo](https://cloud.githubusercontent.com/assets/6086409/4687868/1667a89c-5660-11e4-8b58-df8b600f78ca.png)


    SCLAlertView *alert = [[SCLAlertView alloc] init];
    
    UIColor *color = [UIColor colorWithRed:65.0/255.0 green:64.0/255.0 blue:144.0/255.0 alpha:1.0];
    [alert showCustom:self image:[UIImage imageNamed:@"git"] color:color title:@"Custom" subTitle:@"Add a custom icon and color for your own type of alert!" closeButtonTitle:@"OK" duration:0.0f];