This fixes a lot of stacktraces of socket.error on Windows where EAGAIN != EWOULDBLOCK

in gevent.ares module:
 - rename ares_channel to channel
 - rename ares_strerror() to ares.strerror()
 - remove _strerror()
 - do not use gevent_handle_error C function, use loop.handle_error
 - do not use loop and io watcher definitions (use Python calls instead of fast vtab calls)

add python.pxd with a few common python functions
do not include c-ares stuff, compile it separately
add inet_ntop.c to gevent/ since it's not exported by c-ares and we use it

make ares extension optional - retry setup.py command if it failed

--HG--
rename : gevent/cares.pxi => gevent/ares.pyx

use different callback for each watcher, because layout is not always the same (e.g. timer on windows)

also:
 - incref callback and loop before calling a callback
 - fix compilation problem on visual c
 - do not add cdef functions in watchers, thus removing vtab pointer

resolver_ares: destroy() should be run in the loop, since it calls the outstanding callbacks directly