Use setImmediate to fix stack overflows in eachSeries.

async.eachSeries causes stack overflows for large lists.

    process.version; // => "v0.10.4"
    var async = require("async");
    var _ = require("underscore");
    
    var count = 0;
    async.eachSeries(_.range(10000), function(n, callback) {
        count += n;
        callback();
    }, function(err) {
        console.log(count);
    });

Before:

    RangeError: Maximum call stack size exceeded

After:

    49995000