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.
Without the user specifying an output dir we can only guess where the output files are relative to the source files, right? IIRC, that was the final problem to be fixed in the sourcemap issues.