use Btapp.func and jQuery.Deferred objects to simplify tracker scraping

Btapp.scrape.func.js

/**
Usage:
scrape(hash).then(function(infos) {
    //we have the scrape info for our favorite trackers
});
**/
var btapp = new Btapp;

var scrape = (function() {
    // First off get a function wrapper that we can call at any time
    var func = btapp.func('tracker', 'scrape');
    // It's not really necessary to have all these, but udp is cheap
    var trackers = [
        'udp://tracker.openbittorrent.com:80',
        'udp://tracker.ccc.de:80',
        'udp://tracker.istole.it:80',
        'udp://tracker.publicbt.com:80'
    ];
    // I don't want to worry about destroying the trackers if I
    // don't keep track if I've already asked about this hash
    return _.memoize(function(hash) {
        var scrapes = [];
        // We'll make a request to each of our favorite trackers,
        // then bundle them all the resulting deferred objects
        // using jQuery.when
        _.each(trackers, function(tracker) {
            var scrape = new jQuery.Deferred;
            func({
                hash: hash,
                tracker: tracker,
                callback: function(data) {
                    scrape.resolve(data);
                }
            });
            scrapes.push(scrape);
        }, this);
        // The best part is that memoize will just return the
        // pre-resolved deferred object if we've already made
        // a request for this hash
        return jQuery.when.apply(this, scrapes);
    });
}());