Test and document binding protected member functions

[dean0x7d]

Resolves #991.

This isn't a new feature, it just documents a pattern for exposing protected member functions as discussed in #991.

The test revealed a bug on MSVC 2015 (fixed already in 2017):

struct A {
    void foo() { return 42; }
};

struct B1 : A {};
struct B2 : A { using A::foo; };

static_assert(std::is_member_function_pointer<decltype(&B1::foo)>::value, "");
static_assert(std::is_member_function_pointer<decltype(&B2::foo)>::value, "");

The second static_assert fails only on MSVC 2015. I've noted a workaround in the docs.