Get sass caching working

- Updated to new gulp-intermediate that allows specifying a persistant temp
  folder. Turns out this doesn't matter for caching, but removes some waste and
  i/o.
- Make UUID compiling dir permanent. Seemed like adding a new dir every time was
  invalidating the cache. No need to generate on every run either, would rather
  remove a dependency.
- Place .sass-cache in the user's gulp directory. Would work if it persists in
  the tmp directory, but it's probably better to give the user access to it in
  case they want to delete it.

Tested by copying bourbon into fixtures and including in a couple places. Clean runs with no .sass-cache were consistently ~3.5sec, subsiquent runs were ~2.1sec.

Fixes #54.
Speed up sass command

Running execSync on a Runy command was taking ~1.5 seconds on my computer. TOO
SLOW. Replace with catching and firing errors on the Sass command event emitter.

It's a little less clean, but I think it will cover in most situations.
Neaten some indentation