Refactor the chrono cast functions into chrono.h.

Add unit tests and documentation for the chrono cast.

tests/test_chrono.cpp

@@ -0,0 +1,53 @@
+/*
+    tests/test_chrono.cpp -- test conversions to/from std::chrono types
+
+    Copyright (c) 2016 Trent Houliston <trent@houliston.me> and
+                       Wenzel Jakob <wenzel.jakob@epfl.ch>
+
+    All rights reserved. Use of this source code is governed by a
+    BSD-style license that can be found in the LICENSE file.
+*/
+
+
+#include "pybind11_tests.h"
+#include "constructor_stats.h"
+#include <pybind11/chrono.h>
+
+// Return the current time off the wall clock
+std::chrono::system_clock::time_point test_chrono1() {
+    return std::chrono::system_clock::now();
+}
+
+// Round trip the passed in system clock time
+std::chrono::system_clock::time_point test_chrono2(std::chrono::system_clock::time_point t) {
+    return t;
+}
+
+// Round trip the passed in duration
+std::chrono::system_clock::duration test_chrono3(std::chrono::system_clock::duration d) {
+    return d;
+}
+
+// Difference between two passed in time_points
+std::chrono::system_clock::duration test_chrono4(std::chrono::system_clock::time_point a, std::chrono::system_clock::time_point b) {
+    return a - b;
+}
+
+// Return the current time off the steady_clock
+std::chrono::steady_clock::time_point test_chrono5() {
+    return std::chrono::steady_clock::now();
+}
+
+// Round trip a steady clock timepoint
+std::chrono::steady_clock::time_point test_chrono6(std::chrono::steady_clock::time_point t) {
+    return t;
+}
+
+test_initializer chrono([] (py::module &m) {
+    m.def("test_chrono1", &test_chrono1);
+    m.def("test_chrono2", &test_chrono2);
+    m.def("test_chrono3", &test_chrono3);
+    m.def("test_chrono4", &test_chrono4);
+    m.def("test_chrono5", &test_chrono5);
+    m.def("test_chrono6", &test_chrono6);
+});