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.
diff --git a/docs/changelog.rst b/docs/changelog.rst
index 8b7047d..b0d958d 100644
--- a/docs/changelog.rst
+++ b/docs/changelog.rst
@@ -15,6 +15,9 @@
   for non-MSVC compilers).
   `#934 <https://github.com/pybind/pybind11/pull/934>`_.
 
+* Added support for write only properties.
+  `#1144 <https://github.com/pybind/pybind11/pull/1144>`_.
+
 v2.2.1 (September 14, 2017)
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