Spelling correction in xapian bridge

Xapian bridge now performs spelling correction
on a users query and returns the correctedQuery
(which could be an empty string) as part of the
JSON response to client.

[endlessm/eos-sdk#1982]
we shouldn't change the result API, only add to it IMO. I think it still makes sense to return just a `results` and `numResults` property, but also include a `correction` property which contains its own entire result object (along with the corrected string). something like:

```
{
  results: ['foo', 'bar'],
  numResults: 2,
  correction: {
    correctedQuery: 'baz',
    numResults: 1,
    results: ['hi']
  }
}
```