Change request params and http method

This fixes #5
* The http request to the main endpoint has been changed from `GET` to `POST` 
* The `:format` option is now supported
* Two additional `:metadata` options have been added (`checksum`, `multimedia`)

The documentation has also been updated with new options from ([Endpoint documentation](http://filepreviews.io/docs/endpoints.html)).

This will be released shortly as version v1.2.0
@jpadilla