This has the following advantages:

1. replacing core with another implementation should be easier. I had
   to remove these constants while playing with a libev
   implementation, they use different constants.

2. one can implement deferred imports.

3. the core module is now self contained, i.e. one can "import core" from in

core.pyx: fix bug in getaddrinfo_request.cancel(); previosly evdns_getaddrinfo_cancel was not called properly

add gevent/socketmodule.c and gevent/socketmodule.h necessary to build it; also make corresponding updates to setup.py

- libevent1.4 currently does not compile
- interface is not compatible with previous version
- core no longer initializes the event base (Hub does it)
- core.event and core.http accept event_base
- core.event clears 'callback' and 'arg' after callback is executed or it was cancelled (unless it was persistent). this helps to avoid creating GC cycles
- core: read_event/write_event/timer/active_event/.. are gone (but now available as methods on event_base, with different interface, however)