index: add `setImmediate()` fallback to `process.nextTick`

For v0.8 compatibility. Yes, `co` is indeed usable with v0.8
using things like `gnode` and/or `facebook/regenerator` :)
Merge pull request #105 from TooTallNate/update/add-v0.8-setImmediate-fallback

index: add `setImmediate()` fallback to `process.nextTick`