Gulp intermediate rewrite, fixes 53, 54, and 55

Could use some code review here. Special note:

- sourcemap path change / regression in 2514808
- speed up Sass command: 63c396f. Cool/not cool?
- tests: how much to test, how little?

All comments welcome.
Leet me see what I can do about good error handling. I don't want to add the which on every run because it adds a some time to the task whether everything is working correctly or not. IIRC, on my machine it was around 1.5 sec.