Much more efficient generation of function signatures, updated docs

This modification taps into some newer C++14 features (if present) to
generate function signatures considerably more efficiently at compile
time rather than at run time.

With this change, pybind11 binaries are now *2.1 times* smaller compared
to the Boost.Python baseline in the benchmark. Compilation times get a
nice improvement as well.

Visual Studio 2015 unfortunately doesn't implement 'constexpr' well
enough yet to support this change and uses a runtime fallback.

example/example11.cpp

@@ -14,4 +14,5 @@ void kw_func(int x, int y) { std::cout << "kw_func(x=" << x << ", y=" << y << ")
 void init_ex11(py::module &m) {
     m.def("kw_func", &kw_func, py::arg("x"), py::arg("y"));
     m.def("kw_func2", &kw_func, py::arg("x") = 100, py::arg("y") = 200);
+    m.def("kw_func3", [](const char *) { }, py::arg("data") = std::string("Hello world!"));
 }