fixed deleting meta

delete_post_meta couldn't be reached, so meta field still existed in
database as empty string
fixed checkbox overide

without setting $new to false (or '') the previous value is set as new
(previous ID), wich will result in f.e. a checkbox getting a value of
the previous field (if the previous field is set to 1 the checkbox never
gets unchecked).
Merge pull request #37 from RobVermeer/master

Fixed deleting meta