Provide a way to serve the syndicated tool from an alternative host

# NOTE if you review this please don't merge yet, I'm considering an alternative solution.

Provides a new form field in `widgets.html` that lets you specify an alternative host to fetch the syndicated tool from.

UI looks as follows:

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![screen shot 2014-12-16 at 17 34 53](https://cloud.githubusercontent.com/assets/306583/5458425/0eabe21c-854a-11e4-98c7-cdb2925a5702.png)

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