cxx extension building

I just finished a major rewrite of backend agg using the pycxx
extension tools. Last week I presented weave to a CS class that I'm
taking, and encountered cxx (which weave uses). I had seen this long
ago, but hadn't looked closely at it since.

Given the memory leak in agg that was discovered on matplotlib-users
last week, I decided it would be easier and cleaner to use some good
extension building code rather than hunt it down. The result is a
much cleaner and readable backend_agg. There may be some additional
small leaks to clean up (eg in image or ft2font) but if so they are
likely not major. I was able to generate 2000 figures in a test run
with only a small increase in memory (4% -> 8% on my system). As I
get time I hope to do the image module and/or backend_gtkagg and/or
ft2font.

In any case CXX is very nice - it's committed to CVS - you may want to
test build on your platforms of choice.

Cheers,
John Hunter