lib: Move PostActionValues into post.php

POST variables are superglobals, so lets define them up in the toplevel
POST file.
post.php: Add a value array to PostVariables

Issue #95.
post.php: Add pip_post_is_valid() function

Issue #95.
post.php: pip_post_fail_if_not_valid()

Issue #95.
post.php: Post functions check for valid values

Issue #95.
session.php: Add value array to SessionVariables

Issue #95.
session.php: Perform validity checks on variable names

Issue #95.
log: Add notes for Friday