Issue 1137: pyexpat patch for changing buffer_size

Hello!

Sometimes people have big amounts of text/data in xml files. To make
processing more effective, they should be able to change the buffer size
for collecting character data in one string.

Here comes a patch that applies necessary changes in setattr method. It
handles reallocation sufficiently efficient without introducing extra
variables to the parser class.

Also a documentation patch and some test cases are available.

These patches and tests were done with debian etch python2.5, which is
python-2.5.1 . This patch also works on python2.4 .

Here's a simpler, but still untested version of the patch that cuts out 
the various cases for reallocating when there's already buffered data
and just does a flush every time.

Here's another update of the simplified version, that also updated the
documentation patch for reST and updates the test for the current code.

Committed to the trunk in rev. 59853.

AchimGaedke: thanks for your contribution!  Also, please submit a signed
copy of the
contributor agreement when you can: see
http://www.python.org/psf/contrib/ for details.

Also the shortend version will bring better performance.
Thanks!