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/test_modules.py

@@ -59,4 +59,5 @@ def test_pydoc():
     import pybind11_tests
     import pydoc
 
+    assert pybind11_tests.__doc__ == "pybind11 test module"
     assert pydoc.text.docmodule(pybind11_tests)