Add Sass.load_paths entry to help absolute @import paths

Nice feature added in Sass 2.3 - see nex3/sass@5c93cbfe

This should resolve thomas-mcdonald/bootstrap-sass#259 better than making users manually adding a load_paths entry.
Merge pull request #260 from trisweb/tristan_259_add_global_sass_load_path

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