Feature: Add HTTP timeout option. (#114 by @Rakdar and @clue)

This now respects PHP's default_socket_timeout setting (default 60s) as a timeout for sending the outgoing HTTP request and waiting for a successful response and will otherwise cancel the pending request and reject its value with an Exception. You can now use the timeout option to pass a custom timeout value in seconds like this:

$browser = $browser->withOptions(array(
    'timeout' => 10.0
));

$browser->get($uri)->then(function (ResponseInterface $response) {
    // response received within 10 seconds maximum
    var_dump($response->getHeaders());
});

Similarly, you can use a negative timeout value to not apply a timeout at all or use a null value to restore the default handling.

Improve documentation for withOptions() and add documentation and example for HTTP CONNECT proxy. (#111 and #115 by @clue)

Refactor Browser to reuse single Transaction instance internally which now accepts sending individual requests and their options. (#113 by @clue)