[Map] Provide model of Foldable, improve docs and add insert method.

Fixes #41

example/map.from_record.cpp

@@ -0,0 +1,44 @@
+/*
+@copyright Louis Dionne 2015
+Distributed under the Boost Software License, Version 1.0.
+(See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
+ */
+
+#include <boost/hana/assert.hpp>
+#include <boost/hana/config.hpp>
+#include <boost/hana/map.hpp>
+#include <boost/hana/pair.hpp>
+#include <boost/hana/string.hpp>
+#include <boost/hana/tuple.hpp>
+
+#include <string>
+using namespace boost::hana;
+
+
+//! [main]
+struct Person {
+    std::string name;
+    int age;
+
+    struct hana { struct members_impl {
+        static BOOST_HANA_CONSTEXPR_LAMBDA auto apply() {
+            return make<Tuple>(
+                make<Pair>(BOOST_HANA_STRING("name"), [](auto&& p) {
+                    return p.name;
+                }),
+                make<Pair>(BOOST_HANA_STRING("age"), [](auto&& p) {
+                    return p.age;
+                })
+            );
+        }
+    };};
+};
+
+int main() {
+    Person john{"John", 35};
+    BOOST_HANA_RUNTIME_ASSERT(to<Map>(john) == make<Map>(
+        make<Pair>(BOOST_HANA_STRING("name"), "John"),
+        make<Pair>(BOOST_HANA_STRING("age"), 35)
+    ));
+}
+//! [main]