update Summer of Code links/info

The current web page

http://www.postgresql.org/developer/summerofcode

links to

http://code.google.com/summerofcode.html

which actually gets forwarded to the 2008 SoC page. The 2010 page is at

http://socghop.appspot.com/

and the list of accepted projects is at

http://socghop.appspot.com/gsoc/program/list_projects/google/gsoc2010

Please update this so that it's easier/possible to see what is currently
happening around PostgreSQL in the SoC.