Fix encoding json with Attachments

[jdve]

When using Attachments, we need to properly encode them into json to
share them outside. OutputToJSON handled this correctly, but both
MfgInspector and StationServer did not. Move TestRecordEncoder into
openhtf.util.json, so that it can be shared by all three. For
station_server, patch the tornado json encoder, so that all values being
sent to the client through tornado are properly encoded.

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[jdve]

@googlebot I signed it!

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[google-cla]

Thanks for your pull request. It looks like this may be your first contribution to a Google open source project (if not, look below for help). Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

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[jdve]

@googlebot I signed it!

[jdve]

Thanks @arsharma1 for the feedback! I've updated the PR as your requested.