tweaks so compass-core assets are detected and included in load path

[srerickson]

Not sure if this is the correct approach, but it gets compass working for me

[tdreyno]

Awesome. I was going to do a sprint on this extension this weekend. I'll confirm this works then and merge. Thanks!

[srerickson]

Actually, after more thinking, I'm not sure this change makes sense: append_paths_from_gems will detect assets in gems that might declare themselves, causing duplicate entries in the load path. For example, susy registers itself (for some reason Compass is not defined when that happens for me, so it uses SASS_PATH); so append_paths_from_gems adds it to the load path a second time
... still it's strange that compass's self-registration doesn't work and we need to grab it this way.