Add parameter-less constructors for Bin/Pipeline

gstreamer-sharp/Bin.custom

@@ -1,3 +1,5 @@
+public Bin () : this (null) {}
+
 [DllImport("libgstreamer-0.10.dll") ]
 static extern bool gst_bin_add (IntPtr raw, IntPtr element);
 

gstreamer-sharp/Makefile.am

@@ -82,6 +82,7 @@ customs = 			\
 	AppSink.custom		\
 	AppSrc.custom		\
 	Bin.custom		\
+	Pipeline.custom		\
 	Buffer.custom           \
 	Bus.custom		\
 	Caps.custom		\

gstreamer-sharp/Pipeline.custom

@@ -0,0 +1 @@
+public Pipeline () : this (null) {}