title: How can I invoke a class method by NSInvocation?
body: <p>We can get <code>NSMethodSignature</code> by <code>+ (NSMethodSignature *)instanceMethodSignatureForSelector:(SEL)aSelector</code> of <code>NSObject</code>. Then construct <code>NSInvocation</code> by <code>+ (NSInvocation *)invocationWithMethodSignature:(NSMethodSignature *)signature</code></p>
  
  <p>Here is the problem. We can only get the method signature of <em>instance</em>，how about the class method?</p>