Add support for new params to `push_content` method.

It seems like there are parameters in the [Content API][cnapi] which are not currently supported. This is my initial attempt to add support for the following parameters:

- images
- expire_date
- description
- location
- price
- author
- site_name
- spider


[cnapi]: http://getstarted.sailthru.com/api/content "Content API"