Forward-port the handling of addition and subtraction in the same section from buildout 1

See http://svn.zope.org/zc.buildout/trunk/?rev=127336&view=rev; this fix was made after buildout 2 was branched off.
Update tests for addition and subtraction in same section.

Forward port of http://svn.zope.org/zc.buildout/trunk/src/zc/buildout/buildout.txt?rev=127336&view=diff&r1=127336&r2=127335&p1=zc.buildout/trunk/src/zc/buildout/buildout.txt&p2=/zc.buildout/trunk/src/zc/buildout/buildout.txt
Mention the changes in the changelog
Fix whitespace issues (thank you github editor).
Clean up manual merge changes; tests now pass again.
Merge w/ current master.
Merge branch 'master' into mjpieters-patch-1