Replacing PHPass with password_compat

Per https://github.com/owncloud/core/issues/10050#issuecomment-51333177

also https://github.com/owncloud/3rdparty/pull/110
How will this ever work? All hashes need to be migrated - as a result we need to ship password_compat AND phpass with the next major version and offer a proper migration pass for all apps as well.