New: eq() API method to reduce an API instance to just a single context
and result set for the plural methods.

Fix: Row details events were itterating over non-DataTables rows
creating a Javascript error

Fix: Example - Server-side processing row details example updated to be
able to restore the details row on a redraw.

Update: Documentation - Most examples which used `flatten()` are more
correct to use the new `eq()` method, so they have been updated.

- This set of changes is based on the discussion in thread 19377. While
  working on the fix, I realised that the use of flatten() is too broad,
  so the new `eq()` function is introduced. It is similar to the jQuery
  eq() method in that it reduces the instance to just the selected
  index, although in DataTables this is both the context and the result
  set. Its a small addition, but I think it will provide to be very
  useful