Remove using directive from RazorAHRS.h

In C++11, `std::tr1::function` is now also available as `std::function`.
To avoid the ambiguity, you should remove the directive `using namespace std::tr1`.

And it is also considered harmful, especially if you do it in a header file, because everyone who includes your header, will also have all the names in the global namespace and this may lead to further problems.

The attached patch removes both `using namespace std::tr1` and `using namespace std`.