Rename examples object to example_info in response

Previously, responses containing facet examples had a "examples" object,
which contained a key called "examples" which actually contained the
examples.  This was confusing and error prone.

Instead, name the outer object "example_info", and just name the inner
list of the actual examples "examples".  Something like:

    {"example_info": {"total": 5, "examples": [ {...} ] } }
Merge pull request #269 from alphagov/improve-facet-examples

Rename examples object to example_info in response