Added write only property functions for issue #1142 (#1144)

py::class_<T>'s `def_property` and `def_property_static` can now take a
`nullptr` as the getter to allow a write-only property to be established
(mirroring Python's `property()` built-in when `None` is given for the
getter).

This also updates properties to use the new nullptr constructor internally.

docs/classes.rst

@@ -155,6 +155,9 @@ the setter and getter functions:
             .def_property("name", &Pet::getName, &Pet::setName)
             // ... remainder ...
 
+Write only properties can be defined by passing ``nullptr`` as the
+input for the read function.
+
 .. seealso::
 
     Similar functions :func:`class_::def_readwrite_static`,