Replacing PHPass with password_compat

Per https://github.com/owncloud/core/issues/10050#issuecomment-51333177

also https://github.com/owncloud/3rdparty/pull/110
This patch does not completely remove the use of passwordsalt. E.g.
`./apps/files_external/lib/config.php:		$cipher->setKey(\OCP\Config::getSystemValue('passwordsalt'));`
matches.

This is likely a consequence of https://github.com/owncloud/core/pull/7794#issuecomment-38158797