Fixes #80, which is still an issue in environments with process.nextTick but no setImmediate (e.g. browser with polyfilled Node process).

New order of preference as follows:

  1. setImmediate
- 2. process.nextTick
+ 2. scheduler.postTask
  3. setTimeout

scheduler.postTask seems to be the standardized browser API with the closest performance characteristics and ergonomics to setImmediate, but it's not yet implemented in all browsers (notably Safari support is still missing).