SimpleSAML_Logger: Fix log levels on Windows.

PHP on Windows does not differentiate between all the different log
levels, which means that attempting to specify the 'logging.level'
option will not work properly. This patch replaces the use of the
syslog constants with custom constants.

Thanks to Ryan Panning for fixing this bug!

git-svn-id: https://simplesamlphp.googlecode.com/svn/trunk@2981 44740490-163a-0410-bde0-09ae8108e29a