Including theme partials in a way that supports symbolically linked directories

Including theme partials in a way that supports symbolically linked directories. This closes #1937

I fixed all the JSLint errors being reported by `grunt validate` but I'm not sure if everything's passing now. Excuse my unfamiliarly with node, I'm not a NodeJs developer, just making a blog!

The tests end the same way that a vanilla checkout does:

```bash
Running "express:test" (express) task
Starting background Express server
Ghost is running in testing... 
Listening on 127.0.0.1:2369 
Url configured as: http://127.0.0.1:2369 
Ctrl+C to shut down

Running "spawn-casperjs" task
Fatal error: 
PROMPT>
```

Since it's behaving the same as `master`, I figure I'd throw it over the wall again.

Thanks for your patience on the first pull request