Fix for issue #46

ppgen will detect .il commands with w= values that do not end in px or %
and issue a warning message for each command that fails this validation.
    
New warning message: image width (w=) does not end in px or %. The image will not display
properly:\n {<original source statement here>}
    
(Also: added new warning message for a .il missing both w= and ew=, and
assumed ew=50% in that case to allow generation to continue rather than
getting a Pyton exception.)