Gst::Buffer: wrapped a few missing methods

        * gstreamer/src/buffer.ccg: there is a few methods, where user
          loses object ownership. It can't be done using gmmproc, so this
          methods are written by hand.
        * gstreamer/src/buffer.hg: wrapped a few methods using
          _WRAP_METHOD macro, wrote declarations for a handwritten
          methods.
        * test/test-buffer.cc: a few tests which prove Gst::Buffer class.