Round-trippable exhibit JSON export/import

fixes #416 
ref #391 

![screen shot 2014-03-03 at 17 46 30](https://f.cloud.github.com/assets/111218/2316875/d427414c-a33e-11e3-97cb-7f005e55f83f.png)
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![screen shot 2014-03-03 at 17 46 25](https://f.cloud.github.com/assets/111218/2316876/d43bf7a4-a33e-11e3-91a5-2aae2aa2d8ee.png)

(@ggeisler -- when thinking about what to do with the delete button, maybe we should apply the same treatment here..)