Rewrite spl_autoload() in PHP, support namespaces

Summary: - support namespaces
 - no need for it to be in C++
 - the C++ version didn't actually work when invoked via the autoloader or spl_autoload_call()

fixes facebook/hhvm#2729
fixes facebook/hhvm#2308

Reviewed By: @ptarjan

Differential Revision: D1344838
bump to 3.1.1