Add UIView category to easily calculate CGRect for ui elements on screen (Removed Demo Project)

With this category, you can easily get the CGRect for ui elements on screen when defining coachmarks. Also added a project to demo the usage of this category.

## Usage Example
     UIButton *button = ...;
     NSArray *coachMarks = @[
             @{
                 @"rect": [NSValue valueWithCGRect:[button rectToMarkInView:self.view]],
                 @"caption": @"This is some awesome button!"
             }
         ];
 
     WSCoachMarksView *coachMarksView = [[WSCoachMarksView alloc] initWithFrame:self.view.frame coachMarks:coachMarks];
     [self.view addSubview:coachMarksView];
     [coachMarksView start];