Switch cwd to the gulpfile directory

This makes the intermediate files the special files, and *may* make other Sass
functionality like loadpaths and sourcemaps work a little closer to raw Sass,
once paired with the next commmits.
Add loadPaths for each intermediate file

0.5.0 processes each file separately, adding a loadPath for it so it acts like
it's being processed in place. This re-implements the feature but for all the
files being processed by update at once.

We get this information by using a version of gulp-intermediate that allows
access to the original vinyl objects. Not sure how holding those in memory is
going to affect performance on low powered devices, but for people processing
100s of kb of Sass that's probably the least of their worries.

Testing and probably refinement is necessary, but let's make this available and
see if this alleviates some of the pain users are feeling.
Clean up stderr and stdout logging

Remove our directories so it looks more like the standard Sass output. Adds a
little obfuscation but is more parseable for the user.
Update readme with new behavior
Normalize all Windows paths to unix style slashes

Sass prefers unix style slashes. https://github.com/sass/sass/issues/1328
Cleanup after loadpath and cwd additions

A little renaming, comments, refactoring repeated code into functions and vars,
moving remaining require for rewriteSourcemapPaths inside the function.