New: Number formating helper

- I've come accross a lot of cases recently where numbers need to be
  formatted for display, so I wanted to introduce rendering helpers for
  DataTables to make this easier. Only one helper introduced here -
  `$.fn.dataTable.render.number()` which will return an object that can
  be used by `columns.render` to format "raw" numbers.
Update: The `_` option for columns.render as an object is optional now

- Previously when using columns.render as an object, you had to supply
  an `_` option which was the fallback. However, I've now made it so
  that if there is no `_` option it will just use the raw data from
  `columns.data`
Fix: Fix display issues with HTML5 attributes example