Sort of fixes #6

There is, afaik, no proper way of locking the menu admin to just one level.
However, in the theme can be setup to either strip away all sub-items
(http://d.pr/i/wolF) or show them all at the same level (almost: http://d.pr/i/ff2z).
Merge pull request #14 from mrManner/fix/main-menu-levels

Sort of fixes #6