"Enable Gridder for Admin Pages" should respect "Use administration theme for content editing"

When the gridder is disabled on admin pages, and "Use administration theme for content editing" is turned on (in admin/themes/admin), the gridder shouldn't appear on node add/edit forms. This patch fixes that. It also removes a redundant check for == 'admin' and flips the logic so it exits hook_init when it doesn't apply (just a little easier to understand IMO). I un-indented the code below that (no longer inside the if brackets) so the patch looks bigger than it is.
- Ben