Add sort options to Content Rest API

The output of the documents service of the content rest api is by default ordered by the hippostdpubwf:publicationDate. Add the option to the api to be able to specify a property to sort on and to determine if the sorting should be done ascending or descending.

The query parameter _orderBy specifies the property name, or multiple, separated by commas.
The query parameter _sortOrder specifies the sortorder and can accept the values 'ascending' and 'decending'. In case multiple properties are used for sorting, _sortOrder can contain multiple values as well, for example: http://localhost:8080/site/api/documents?_orderBy=newsdocument:title,newsdocument:date&_sortOrder=ascending,descending

If sorting on a specific property is done, the resultset should only contain documents that have that property.