Update accessTokenUrl and add getter-setters

The retrieval of access tokens does not work with the FlattrAuthenticator due to an erroneous accessTokenUrl. Unfortunately this needs a code change. Commit description see below.

The accessTokenUrl was updated to https in order to match the one defined at http://developers.flattr.net/api/#authorization.
Further a getter-setter pair was added in order to allow derived classes and implementing application to change the URL if necessary.
Further a getter-setter pair was added for the request token url in order to change the url if necessary without the need for a binary flattr4j release