Make JSON.parse unwrap and use the "high" value in a FacetedValue

To use JSON from untrusted sources, we would have to unwrap the
FacetedValue. If we didn't, a call to JSON.parse with a FacetedValue
would return a FV with its high/low values evaluated by JSON.parse,
<P ? JSON.parse(high) : JSON.parse(low)>
Add an example that exercises the FacetedValue unwrapping in JSON.parse
Merge pull request #3 from btlachance/untrusted-json

This looks good to me.