More allDocs params

To allow users more flexibility when querying all documents, endKey, skip and descending have been added as optional parameters to the method, with unit tests before and after the new functionality to assert expected behaviour and backwards compatibility.

This may be a fix for request https://github.com/doctrine/couchdb-client/issues/24

Happy to make amends if you feel that I've strayed from the coding style or proposed functionality of the library.