export and import constrains

[pbauer]

Fixes #65

[mauritsvanrees]

Tested with export and import on Plone 4.3, and it works. Thanks! At first I thought we would miss the constrainTypesMode field itself, but this is automatically contained in the schema.

One minor inline comment, but is also fine to merge as is.

[pbauer]

Thanks for the review. I now already had a case where the constrains didi not allow importing the exported data, e.g. because a folder or a collection was added and then later disallowed.
How about adding content disregarding the constraints e.g. using plone.dexterity.utils.createContentInContainer or Products.CMFCore.TypesTool._constructInstance?

[mauritsvanrees]

Ah. Yes, that seems good.