Table.remove_columns on a string

someone at the SciPy tutorial ran into trouble by trying to call `.remove_columns` on a single column, rather than `.remove_column`.  The result of `.remove_columns('foo')` on a string is to try to iterate over each character in the string as a column name.

I think for strings we should do like Pandas--treat it as a comma-separated list of colnames.  Granted this breaks in bizarro case where one of the column names contains a comma. But in that case we can throw up an error, maybe, or just not support it.  For most cases I think it could be useful and would prevent this common mistake.