Renamed new to newVal so it works in c++ projects

Hi Martin,

In MTKObserver.h there are a few block typedefs that use ```new``` as parameter name for the new value of observed property.  Block-KVO fails to compile with projects that use C++ because ```new``` is a reserved keyword in C++. This pull request contains updated MTKObserver.h to use ```newVal``` instead of ```new```.

Thanks,
Rahul