Old API: Mark $ method as deprecated
API: Selectors - 'filter' selector modifier renamed to be 'search'

- Renaming in keeping with the new terminology
- Updating example which uses it (main documentation still to be
  written)
- Backwards compatible with a bit of logic to check if the old version
  is being passed in
Update - init options: Rename displayLength to pageLength for consistency

- The displayLength option is poorly named, particularly with the new
  API refering to it as the page length (page.len()) so, using the new
  translation option to allow backwards compatiblity while using new
  parameter names, I've updated this parameter's name and its
  documentation.