Rename sort parameter to sort_by

It was sort_on in the feature test, sort in the code and sort_by in the
wiki. The wiki rules!

@pbadenski
@robyoung
Add basic query sorting to Bucket

@pbadenski
@robyoung
Fix query sort feature test

- Explicitly expect an integer, by default the numbers are parsed as
  strings
- Extract nth result position parsing in step definitions
- Add 'first' and 'last' options for result position parsing.

@pbadenski
@robyoung