Fix .sass-cache location, switch to cwd + .sass-cache

This change creates .sass-cache in the user's gulpfile directory. Previously it
was created in the gulp-ruby-sass node module directory.
Make path to source files relative

Adds a sourcemapPath option for specifying the relative path from CSS to source files
Normalizes Windows paths to Unix/uri style with slash().
Tested with many combinations in browser. :ok_hand:.
Improve sourcemapPath documentation