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