Detect the absence of setImmediate for nodejs-v0.8 and lesser

And fallback to process.nextTick.

Here is how it fails on cloudfoundry's v0.8.2 without this patch:
/var/vcap/data/dea/apps/stoic-0-bda8723f9d67b2926c82dcae327b7902/app/node_modules/async/lib/async.js:90
        async.setImmediate = setImmediate;
                             ^
ReferenceError: setImmediate is not defined
    at /var/vcap/data/dea/apps/stoic-0-bda8723f9d67b2926c82dcae327b7902/app/node_modules/async/lib/async.js:90:30
    at Object.<anonymous> (/var/vcap/data/dea/apps/stoic-0-bda8723f9d67b2926c82dcae327b7902/app/node_modules/async/lib/async.js:947:2)
    at Module._compile (module.js:449:26)