Changes the mapinfo so that the mapped data is writable

The Problem is, that in the current state it is not easily possible to edit the buffer data in a gstreamer python element since you get a copy of the reals buffer.

This patch overrides the mapinfo and the function generating it in a way so that mapinfo.data is now a memoryview pointing to the real buffer. Depending on the flags given for this buffer the memoryview is r/w.

Fixing following issue #13 (closed)