This is a first pass and will probably be altered before merging. Still
needs more hooks and more tests. Only implemented for cURL so far.

Trivia: I wrote this completely without internet access (and hence, no
documentation either). Imagine my surprise when it worked first time.

If we end up with a situation where our hook doesn't fire for some
reason, we shouldn't pass this on to the hooks. Instead, wait until the
end when we ensure they're instances.

fsockopen doesn't support multiple simultaneous requests, so we just
fake it here instead. What it actually does is to send every request
sequentially, so the result back from Requests::request_multiple() will
be the same.

curl.before_request and curl.after_request are now fired on every cURL
request. curl.before_send and curl.after_send are for single requests,
while curl.before_multi_add, curl.before_multi_exec and
curl.after_multi_exec are for multiple requests.

curl_multi_info_read()'s 'result' value is the same as curl_errno(),
which we handle per-request in a nicer way.

Rather than relying on typecasting the cURL resource to an integer to
form a key, use the proper key instead.

To make it easier to track a request after it's complete, we pass in the
original identifier along with the response.

This should all be abstracted away.

This is a bit of duplication (given that you can just make your own
Requests_Hooks and add it), but it saves a few extra lines on the user's
end.

This should fix the currently broken unit tests. Yay!

If we're in non-blocking mode, there's no reference to return. We're not
really saving anything with this anyway.

This is currently lacking documentation. Coming soon.

multiple.request.complete is called on the request's hooking system, not
the global one.

Also, move the callback to a proper method. Fixes 5.2 compatibility as well as avoiding duplication.

Previously, this checked a nonexistent variable, and I'm not sure why. If it
ever needs to be fixed, blame me.