Replace PYBIND11_PLUGIN with PYBIND11_MODULE

This commit also adds `doc()` to `object_api` as a shortcut for the
`attr("__doc__")` accessor.

The module macro changes from:
```c++
PYBIND11_PLUGIN(example) {
    pybind11::module m("example", "pybind11 example plugin");
    m.def("add", [](int a, int b) { return a + b; });
    return m.ptr();
}
```

to:

```c++
PYBIND11_MODULE(example, m) {
    m.doc() = "pybind11 example plugin";
    m.def("add", [](int a, int b) { return a + b; });
}
```

Using the old macro results in a deprecation warning. The warning
actually points to the `pybind11_init` function (since attributes
don't bind to macros), but the message should be quite clear:
"PYBIND11_PLUGIN is deprecated, use PYBIND11_MODULE".

tests/pybind11_tests.cpp

@@ -52,8 +52,8 @@ void bind_ConstructorStats(py::module &m) {
         ;
 }
 
-PYBIND11_PLUGIN(pybind11_tests) {
-    py::module m("pybind11_tests", "pybind testing plugin");
+PYBIND11_MODULE(pybind11_tests, m) {
+    m.doc() = "pybind11 test module";
 
     bind_ConstructorStats(m);
 
@@ -61,6 +61,4 @@ PYBIND11_PLUGIN(pybind11_tests) {
         initializer(m);
 
     if (!py::hasattr(m, "have_eigen")) m.attr("have_eigen") = false;
-
-    return m.ptr();
 }