Update zen_id_safe to remove underscores

Since all of the IDs/Classes in Drupal and Zen use dashes ( - ) rather than underscores ( _ ), I believe that the zen_id_safe function should replace underscores with dashes to be consistent.

This change also requires zen_id_safe to be wrapped around the line in which the node-type-* class is defined.

Thoughts?

Patch Summary (all edits are made in zen/template.php):
1) changed line 183 from
$classes[] = 'node-type-' . $vars['type'];
to
$classes[] = zen_id_safe('node-type-' . $vars['type']);
2) Updated the preg_replace to replace underscores.
3) Fixed a line in the comments on line 249. It should read "// If the first character is not a-z, add 'id' in front.
", not "// If the first character is not a-z, add 'n' in front."