Moved uploaded assignment files into a subdirectory called moddata,

Moved uploaded assignment files into a subdirectory called moddata,
where data from other modules can also live later on.

Also added a README for that directory to warn teachers not to mess
with it, version code to perform the upgrade, tweaks to assignment/lib.php
and a tweak to reading module so that assignment files aren't listed
in the list of possible readings (could get messy).