core: make io class similar to all other watchers, use the same callback function for all watchers; make loop and watcher classes public

- the way io calls its callback is changed: it used to pass (events, self) always, now it passes *args like the other watchers do.
  in order to find out which events fired, start the watcher as watcher.start(func, EVENTS, *args) - func will be called as func(<EVENTS FIRED>, args)
- do a type check that callback passed to start() or to watcher.callback.__set__ is a callable;
- same for 'args' attribute, except allow it to be None
- declare some of the methods as cpdef for speed when used in C code
- make '_incref' readonly rather than public
- add cdef methods _start() and _feed() because start() and feed() use *args and cannot be "cpdef"
- update modules affect by API change: gevent.select and gevent.server

slightly change the way errors are handled by loop so that setting 'handle_error' attribute on the Hub is enough

'handle_error' argument for run() method is gone

all watchers now behave like they did in 0.13: the incref themselves while in use by the event loop (that is, _ref versions are kept but without _ref in the name)

one reason for that is that doing ev_watcher_stop in watcher's __dealloc__ crashes: loop attribute is no longer a valid reference in __dealloc__; (although this could probably be worked around)

another reason is to make it simpler, having 2 sets of watchers is complex to explain.

third reason is: using watchers that automatically stopped themselves on __dealloc__ introduced a lot
of incompatibilities and potentially unexpected behaviour compared to gevent-0.13. it is also not recommended in general to depend on CPython's refcounting for resource management.

pywsgi: set SERVER_NAME and SERVER_PORT only once after socket is bound; do not overwrite SOCKET_NAME and SOCKET_PORT provided by the user; do not fail on unix sockets (use '' in that case)

add 'wfile' as a deprecated property

core_.pyx: use <void*> instead of <object>, because cython 0.14.1 will add additional incref/decref for <object> arguments