support appTransformDirs which are symlinks

one of my `appTransformDirs` happened to be a symlink, which caused nothing under it to be transformed, because the dir didnt match. (which in turn caused raw sass to appear in my css)

the fix was:

``` js
appTransformDirs: [
    './app/', './app/bower_components/',
    fs.realpathSync(path.join(__dirname, './app/bower_components/hn-core/app/'))
]
```

which could be done within your lib
