Add 'importPath' option

Added the importPath compass compiler option which allows us to specify custom paths that should be searched when using the @import statement. Allows you to easily include frameworks like foundation by using (is there better way than this?): 

    var app = new EmberApp({
        compassOptions: {
            importPath: [
                process.cwd() + '/vendor/foundation/scss'
            ]
        }
    });