bugfix: Restore support for windows machines.

Due to recent changes to fix the way we were building URLs (mixing what the 'baseurlpath' configuration option and the current URL, see #396), we introduced another bug by assuming file paths will always use slashes ('/'), which obviously is not true in Windows machines. This commit fixes SimpleSAML_Configuration::getBaseDir() and SimpleSAML\Utils\HTTP::getSelfURL() to take that into account.

This closes #414.