Improved error handling.

Hi!

I've added error handling to the middleware. When the compilation fails (error status different than zero) the middleware will raise the exception and this will be propagated to the browser.
This way we behave more like Sprockets and the user doesn't have to check the terminal to see if compilation succeeded.

This settings can be disabled using:

```ruby
GulpRails.options[:hide_errors] = true
```

Also, I've added `GulpRails.options[:args]` to pass argument (like the task to run) to the executable.

Hope this helps!
  Paolo