Cascaded Union Aggregate function

For those people who have unions to do that are slow, feel free to try out
this aggregate union function.

I haven't tested it enough for it to make it into 1.3.4, but have included a
link to the source code in the wiki.  It should work just fine on PostGIS
versions 1.2.2 and above.

http://postgis.refractions.net/support/wiki/index.php?PL%2FPGSQL%20Pseudo%20
Cascade%20Union%20Aggregate%20Function

Let me know if you run into any problems with it.  It has worked well for
the samples I have used it on.

Thanks,
Regina