fixed: will only try and parse css if it exists

Fixes the Type Error when there is no css to compile. Allow you to use a simple gulp task to support live reloading and not have to restart the process.

```javascript
gulp.task('stylus', function () {
  return gulp.src('src/style.styl')
    .pipe(stylus())
    .on('error', function () {this.emit('end')})
    .pipe(gulp.dest('.'));
});
```

Closes #51
Fixes #46, #50