remove obscure green.util module

darcs-hash:20090318102228-b2873-78d63fc4ef66b71d6ea4d0ba77cb5efc56820213
rename modules in green package: engine -> core, session2 -> session; session -> sessionold

- move GreenBase into green.__init__ and add doctrings to it
- fix scripts not to use removed GreenEngine.shutdown method

darcs-hash:20090318102709-b2873-def8491c1d7f8f500f22abf531b889a51b8dfaf6