New: column() API methods for working with a single column

- To round off the API, this commit adds a column() method, for working
  with a single column, as with row/rows, cell/cells and table/tables.

- New methods:
  - column() - Column selector
  - column().data() - Get column data
  - column().header() - Get the column header cell
  - column().visible( set ) - Get / set the column visiblity
  - column().order( dir ) - Order the table by this column

- Note that I haven't implemented column().search() yet as I'm sure
  there must be a better way of dealing with the singular functions...
  Need to look at the four groups a bit more before this (and then the
  API :-) ) is finalised