Take a GMainContext as well, and update the resolution code to dispatch

Take a GMainContext as well, and update the resolution code to dispatch

	* libsoup/soup-dns.c (soup_dns_lookup_resolve_async): Take a
	GMainContext as well, and update the resolution code to dispatch
	each lookup result in the correct context.

	* libsoup/soup-address.c (soup_address_resolve_async_full): New
	method that takes a GMainContext to pass to
	soup_dns_lookup_resolve_async.

	* libsoup/soup-socket.c (soup_socket_connect): Use
	soup_address_resolve_async_full. Fixes a problem reported by Armin
	Bauer.

	* configure.in: update to require glib 2.6, since apparently the
	code does.