Add Sass.load_paths entry to help our absolute @import paths

Add a global `Sass.load_paths` entry for our load path. This should make our @imports rock solid on any Sass 3.2+ installation. Nice feature added in Sass 2.3 - see nex3/sass@5c93cbfe - based on a similar issue and request by a gem, nex3/sass#131

This should resolve thomas-mcdonald/bootstrap-sass#259 better than making users manually adding a load_paths entry, and prevent issues in the future.