#13: Fix watch functionality to work in parallel

This is a follow-up to #48, capturing the full changeset.

@markdboyd, Ccing you here for the continued discussion.
Why do we need a watch:theme task here when theme.js sets up watch tasks for each configured theme? I would think we'd want to pull all the watch tasks in theme.js together under a parallel task.

The biggest problem I see is the target of this watch (themes/**/*.sass...) is different than what the compile-theme task will actually compile.