Use Rack::TryStatic for static public files.

[ntalbott]

Rack::TryStatic will attempt to load paths from public before
passing them on to serve.

Also has more graceful handling for the case of a missing compass
config file.

Should fix #36, but I'd really appreciate review.

[jlong]

Hey @ntalbott. What does this patch give us over just using Rack::Directory? Would moving Rack::Directory to the front of the cascade solve the same problem?

Also, I don't think this will do anything for #36. Unfortunately serve export doesn't load the middleware that you are using from config.ru. We should figure out a way to fix this.

[jlong]

Oh, I see. Rack::Directory needs to go last otherwise you get directory indexes when an index file exists. I've added a version of this in master.