Use both pathwatcher and polling to detect changes more robustly.

This strategy was suggested by @glasser after we realized just how
hopeless it is to probe the file system to test pathwatcher:
https://github.com/meteor/meteor/issues/3285#issuecomment-67296961

I've made some attempt to de-duplicate these events (since we're
effectively watching twice now), but we have other mechanisms for dealing
with bursty file change events, so these measures do not need to be
completely bulletproof.

Fixes #3284 (again).