netlink: attach marshalling to the NetlinkSocket

This makes it possible to use iproute2 w/o threading and complex stuff, and simplifies
integration with other projects. The simplest example::

	from pyroute2 import IPRSocket
	ip = IPRSocket()
	while True:
		print(ip.get())

The NetlinkSocket class behaves like any ordinary socket, but provides also methods
`.get()` and (planned) `.put()`. The IPRSocket class also provides auto-binding
(this functionality probably will be moved to NetlinkClass also).

NetlinkSocket objects can be used in poll/select cycles, as any other socket objects.