Add 'presentation' formatter

Issuu supports different layouts for embedded magazines. The page flip (currently available in the Issuu module) is only one of them. A client of mine wanted to use the 'presentation' formatter, which was done by adding a 'viewMode' => 'presentation', to the flashvars. See the patch.

For future versions, it might be nice if the user can choose the viewMode on a per-node basis instead of per-field.