Fixed GenericInlineMenuItemForm parent spec

In MenuItem model, parent field allows blank/null, therefore
the form field spec for parent in GenericInlineMenuItemForm
needs to include required=False in order to allow menu items
that have no parent to be edited inline (e.g. on pagelets pages
that refer to them) without auto-setting a "random" parent value
on save.
Merge pull request #29 from caktus/parent-not-required

Fixed GenericInlineMenuItemForm parent spec