- this leads to better error information, esp. with easy_install which suppresses stackrace

- web.py only likes sockets that have _sock property

the former is default now

- although not necessary, it should be there for completeness and for the sake
  of potential alternative implementations of greenlet

- do not use Pool as it's already demoed in dns_mass_resolve.py

- the default value is "gevent x.y Python x.y"