fix bad_any_cast_exception.

revision a return value of bad_any_cast_exception::what().
add an explaination of a cause.
fix filename to `nek/any/triats.hpp` from `nek/any/is_any.hpp`
rename member function to `bool is_empty() const` from `bool empty() const`

add `bool empty() const` member function to compatible interface.
support C++11/14 of VC12
supported using alias and default/delete function