New: cell invalidation methods

- New API methods:
  - cell().invalidate() - Invalidate a single cell
  - cells().invalidate() - Invalidate multiple cells

- Note that the invalidation is actually row based, rather than cell
  based - i.e. the whole row is invalidated. This may change in future
  versions of DataTables, but I'm concerned that it would add a lot of
  code for little enhancement.
New: cell set data API method

- New API call:
  - cell().data( set ) - Ability to set data for a single cell. Will
    invalidate the whole row.