Add GSimpleIOStream class

GSimpleIOStream represents an object that wraps an input and an output
stream making easy to use them by calling the #GIOStream methods.

https://bugzilla.gnome.org/show_bug.cgi?id=741630

docs/reference/gio/gio-sections.txt

@@ -1039,6 +1039,20 @@ GIOStreamAdapter
 g_io_stream_get_type
 </SECTION>
 
+<SECTION>
+<FILE>gsimpleiostream</FILE>
+<TITLE>GSimpleIOStream</TITLE>
+GSimpleIOStream
+g_simple_io_stream_new
+<SUBSECTION Standard>
+GIOStreamClass
+G_TYPE_SIMPLE_IO_STREAM
+G_IS_SIMPLE_IO_STREAM
+G_SIMPLE_IO_STREAM
+<SUBSECTION Private>
+g_simple_io_stream_get_type
+</SECTION>
+
 <SECTION>
 <FILE>gfileiostream</FILE>
 <TITLE>GFileIOStream</TITLE>