Fixed #399

If MSGPACK_DISABLE_LEGACY_CONVERT is defined, msgpack::object::convert(T*) is removed.
Added MSGPACK_DISABLE_LEGACY_CONVERT to build system and documents.

Please define MSGPACK_DISABLE_LEGACY_CONVERT and update your code as follows:
Replace
  int i;
  obj.convert(&i); // Removed pointer version
with
  int i;
  obj.convert(i);  // Reference version

test/msgpack_stream.cpp

@@ -45,7 +45,7 @@ const unsigned int kLoop = 1000;
         if (it == vec.end()) goto out;                                  \
         msgpack::object obj = result.get();                             \
         vec_type::value_type val;                                       \
-        obj.convert(&val);                                              \
+        obj.convert(val);                                               \
         EXPECT_EQ(*it, val);                                            \
         ++it;                                                           \
       }                                                                 \