Updating Markdown class + use spl_autoload_register

Updating Markdown class + use spl_autoload_register

spl_autoload_register now calls the GNUsocial_class_autoload function
instead of us replacing the magic __autoload($cls). This means we can
queue up other autoload functions, such as the one now used for extlib
functions which exist directly in the 'extlib/' folder or have proper
namespacing (which our new Markdown class does).