Change Table so assigning to non-existent col name creates new column with that value
Allow for multidimensional data when setting with t['new'] = newcol
Check that item is string when creating a new table column

Previous version broke setting rows with t[1] = (values) because
it tried to create a new column 1.  Instead make sure the item is
a string and is therefore referring to a column name.
Fix problem adding new column to an empty table
Add tests for creating a new column using item setter
Propagate Column metadata when creating column using item setter
Change table setitem so a masked column can be added
Update table docs to use setitem syntax for adding a column
Change from np.array to asarray when setting from non-ndarray item
Merge pull request #726 from taldcroft/table-new-col

Use setitem syntax to create a new Table column