MiniObject: fixed gst_mini_object_make_writable wrapper

gst_mini_object_make_writable unrefs object, but in gstreamermm,
Glib::RefPtr class is used, so we have to make an extra reference
for Glib::RefPtr destructor (where unreference method is called).

	* .gitignore: added miniobject test script to ignored list
	* gstreamer/src/buffer.ccg:
	* gstreamer/src/bufferlist.ccg
	* gstreamer/src/caps.ccg:
	* gstreamer/src/event.ccg:
	* gstreamer/src/message.ccg:
	* gstreamer/src/query.ccg: used MiniObject::create_writable method
	  in create_writable implementation of  derived classes
	* gstreamer/src/caps.hg: Gst::MiniObject as a base class for
	  Gst::Caps, updated comment
	* gstreamer/src/bufferlist.hg:
	* gstreamer/src/event.hg:
	* gstreamer/src/query.hg: updated comment
	* gstreamer/src/miniobject.ccg: fixed create_writable
	  implementation
	* gstreamer/src/miniobject.hg: create_writable as hand-written
	  method
	* tests/Makefile.am:
	* tests/test-miniobject.cc: added Gst::MiniObject test suite