Add support for a "warnings_callback" argument to Rules.match(). If provided the
function definition needs to be:

def warnings_callback(warning_type, message)

The callback will be called with a warning type of yara.WARNING_TOO_MANY_MATCHES
and the message will be a string indicating which rule caused the warning. I
think a warning type and a message is reasonably flexible in case we introduce
other runtime warnings in the future.

If a callback is not provided we print a warning on stderr using the normal
python warning system. It's worth noting the function I'm using was introduced
in python 3.2. I can switch it to something more portable if you don't want to
pull support for 2.x yet.

While I'm here, also chase the renaming of rules_list_head and other list
variables so that it can compile with latest yara master.