- cancel() is usually used in "finally" sections and raising anything here is not helpful

GreenletSet: make spawn* function work well with Greenlet subclasses (do not assume what the arguments should look like)

greenlet: fix spawn_link* functions to work well with Greenlet's subclasses (thanks to Marcus Cavanaugh)

- some module don't have a single unittest; they were missed by --stats

--HG--
rename : doc/gevent.evdns.rst => doc/gevent.dns.rst

--HG--
rename : gevent/evdns.py => gevent/dns.py

testrunner.py: if sqlite3 module is not available do not pass "None" as database name to the subprocesses