add a .data to buffers; it feels more natural to me than doing str(buffer) compared to the other properties.

Original commit message from CVS:

* gst/gst-types.defs:
* gst/gstbuffer.override:
* gst/pygstminiobject.c: (pygstminiobject_new_noref):
* testsuite/test_buffer.py:
add a .data to buffers; it feels more natural to me than doing
str(buffer) compared to the other properties.
make sub_buffer test for data