Support for Compass

To use compass library, we have to hard code compass library path (e.g. `/path/to/gem/ruby/2.1.0/gems/compass-core-1.0.1/stylesheets`)
 to `sass.load_paths`. It's so painful.

So, I add a new configuration option `sass.compass` (default: `false`).

If this option is enabled in unsafe mode, compass libraries are automatically imported.