Prepare for some of the Blaze changes in 0.9.1

This makes all test pass against the current revision (01775074928cfe06f9276ea7cfae5583adad2ab9) of the 'blaze-templates' branch (which will be part of 0.9.1).

There will be another change for 0.9.1, not yet implemented -- ReactiveVar will be its own package rather than in the UI namespace, but I think we'll backcompat it. So maybe this is all that's needed for 0.9.1 support on iron-dynamic-template.

Oh. I haven't fixed `UI.registerHelper('DynamicTemplate', Template.__create__(...))` yet but I guess tests don't exercise that? Or maybe we have backcompat? I'll check.

Next up: look at iron-layout