_.defer and process.nextTick under node.js

Really simple improvement to use `process.nextTick` instead of `setTimeout` under Node.js. Quick benchmarks show that the incurred overhead of the current implementation (using `setTimeout(fn, 0)`) varies between 600 µs to 1.2 ms. With the current improvement, it varies between 200 µs to 450 µs.

After a few years in production, that could potentially save us 1 or 2 seconds! :)