#13: Fix watch functionality to work in parallel

This is a follow-up to #48, capturing the full changeset.
I renamed this from watch:compass to watch:theme, because the compile-theme task is scoped for more than compass and that naming should be more consistent to that point. However, this watch implementation is much more coarse-grained than the implementation of compile-theme's configuration.

If this works for now, I'm in favor of moving forward but I suspect long term we need to set up the various watches in theme.js and parallelize them here based on the steps per theme.