Added support for session.upload_progress as APC replacement

APC is no longer available as of PHP 5.4, Zoph uses APC to track upload
progress. Fortunately, PHP 5.4 also introduced session.upload_progress
to track upload progress. This change makes Zoph automatically select
either of the two ways, depending on the currently running version of
PHP.

fixes #39
Merge branch 'issue#39'