Add path and host settings

Add 2 optional settings to set path and host.  

1. Host is useful if you have a custom CNAME for your assets (e.g. ```cdn.mywebsite.com```). 
2. Path is used if you don't want to store your images at the root of your bucket (e.g. ```/assets```).

Used in combination, you can have your asset urls look like ```http://cdn.mywebsite.com/assets/uuid.jpg```