- store the update environment on 'environ' attribute
- do not set SERVER_NAME and SERVER_PORT if they already were provided by the user
- add get_environ() and use it in the request handler
- add kill() method

server: rewrite StreamServer to use BaseServer and to use a persistent event rather than an accept loop

core: make http_request reply with 500 in __dealloc__ if no reply was sent; make http class responsible for detaching http_requests on connection close;

- make http_request weak-referencable
- remove set_closecb() method from http_connection (it is used internally now, to detach http_requests)
- remove set_gencb() from http class; pass the callback to the constructor instead
- remove set_cb() from http class: it does not fit very well with the current design and is not used by gevent itself; use issue tracker if you need this back

core: when reporting 500 Internal Error, add "Connection: close" and "Content-type: text/plain" headers