add host and public configable

I changed the order of config array merge, 

So that when I want to change the 'host', or 'public' config value, I can set it in the config file and it will overwrite the default one.

This is usefull when the laravel is not installed on web root

for example, my laravel is installed under: http://192.168.1.123/~william/appname/

So I added this to my config file:

```
'host' => url('/'),
```

so that the thumbnails url will work