Reverse order of context/frame lookup (issue #119)

I don't know if there's any particular reason why `contextOrFrameLookup` looks at `context` first, but I think this has been the cause of a lot of heartache surrounding the `{% set %}` block and also macro arguments. If something in the frame overrides something in the context, the context was still being trusted.

The tests pass for me, as well, which I found both pleasant and surprising.