Add signal handling support

- resource uv_signal_init([resource $loop])
- void uv_signal_start(resource $sigHandle, callable $onSignal, int $signal)
- void uv_signal_stop($signalResource)

Example:

    <?php
    const SIGINT = 2;
    $sigHandle = uv_signal_init();
    $onSigint = function($sigHandle, $sigNum) {
        printf("Caught signal: %d\n", $sigNum);
        uv_signal_stop($sigHandle);
        uv_stop();
    };
    uv_signal_start($sigHandle, $onSigint, SIGINT);
    uv_run();
Fix copy/paste error message typo