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".

docs/classes.rst

@@ -27,15 +27,11 @@ The binding code for ``Pet`` looks as follows:
 
     namespace py = pybind11;
 
-    PYBIND11_PLUGIN(example) {
-        py::module m("example", "pybind11 example plugin");
-
+    PYBIND11_MODULE(example, m) {
         py::class_<Pet>(m, "Pet")
             .def(py::init<const std::string &>())
             .def("setName", &Pet::setName)
             .def("getName", &Pet::getName);
-
-        return m.ptr();
     }
 
 :class:`class_` creates bindings for a C++ *class* or *struct*-style data