Fix for uncaught exception on iOS 5 when using class names for block arguments

NSMethodSignature on iOS 5 can not handle quoted class names in objc types and fails with exception:
```objective-c
SLBlockDescription *descr = [[SLBlockDescription alloc] initWithBlock:^(NSString *str){}];
```
```
Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '+[NSMethodSignature signatureWithObjCTypes:]: unsupported type encoding spec '"' in '"NSString"4''
```
So I removed quoted parts from objc type.

And because iOS 5 is far from been mainstream and the fact that the class of the argument may be useful in some cases, I used compile-time check for iOS Deployment Target.