Fix the async issue for StackOverflow API calls

[havanagrawal]

We should probably start using promises/futures for our StackOverflow API calls. Right now, the behavior is really unpredictable (read as "completely wrong")

For instance, the function getLastActivityTime in watcher.js doesn't actually wait for getting the last activity time, and always ends up adding 0 as the last known time.
We could fix this by

1. Passing userId to the function
2. Updating the watcher in getLastActivityTime inside the callback itself, and return nothing

but that's just bad design imho.

Solutions:

1. Use promises/futures
2. Use a better rest client