Restore previous tenant and tenant callback#10

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pupeno wants to merge 4 commits intowireframe:masterfrom

Commits on Sep 18, 2011

Don't forget the previous tenant when going into a with_tenant block.
pupeno
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Commits on Sep 20, 2011

Test resetting the current_tenant to the previous one.
pupeno
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Commits on Nov 7, 2012

When setting a tenant as the current one, call a method in the tenant (a callback).
pupeno
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Make sure the callback is run on current_tenant= as well. And tests.
pupeno
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