POE::XS::Loop::Poll

In the spirit of POE::XS::Queue::Array, I've released
POE::XS::Loop::Poll, an implementation of POE's event loop written in
C.

I haven't done any speed benchmarks (or found any), but I expect it to
be slightly lower overhead than the built-in POE::Loop::Select.

Of course, without a benchmark it can only be a guess.

I'm fairly confident of the code as it stands, but I wouldn't
recommend pushing it into production without testing.

Tony