Also strip the leading slash from the outputPath.

Without this change, on master of ember-cli, compass fails attempting
to access `/assets`
Late-bind compassOptions.

Prior to this commit, whatever compassOptions you pass into the
EmberApp constructor is what the plugin will use. That prevents
usage like this from working:

    var app = new EmberApp();

    // fetch config with environment-specific compassOptions
    var config = app.project.config(app.env);
    app.options.compassOptions = config.compassOptions;

    module.exports = app.toTree();

The add-on will now use the value of app.options.compassOptions
when it is compiling assets instead of when it is initializing.
Merge branch 'strip-leading-slash-from-output'