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.
can you preserve the `which` check. the spawn error when it's not available is really obtuse.