pywsgi: do not add CONTENT_TYPE to the environ dict when there's no Content-Type header in the request

wsgi: fix issue #58: even if http_request instance is leaked, reply with 500 error immediatelly when application raises an error. Thanks to Jon Aslund.

this is needed for websocket which needs to own the connection

pywsgi: ignore errors that happen when socket is closed (for example if it was  already closed by websocket handler)

pywsgi: do not use chunked transfer encoding in case of 304 and 204 responses as it creates a non-empty message body which is against RFC and causes some browsers to fail. Based on the patch by Nicholas Piël.

Note, that zero-length string encoded with chunked transfer-encoding is a non-empty message-body.
That causes some browsers to fail (e.g. safara and firefox on macos)

Thanks to Nicholas Piël.

evdns_base.getaddrinfo: do not accept optional 'arg', it's not needed anywhere
evdns_base: add resolve_ipv4

this makes subclassing the handler easier

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)