Causes now contain a PageRef instead of an ImageRef.

Causes now contain a PageRef instead of an ImageRef.

Fixes #49 

**POST /api/cause_type**
* creates a new cause / updates an existing one

````json
{"name":"homelessness", "page":{"url":"https://www.facebook.com/109613735724487","urlProvider":"FACEBOOK"}}
````

Example source:
https://www.facebook.com/pages/Homelessness/109613735724487

Example usage:
* https://www.facebook.com/109613735724487 - is a link to the page
* GET /109613735724487/picture - gets the primary image