Get data by header name

Added Column#row_data and Row#column_data. 

Wanted to be able to write epxressions like this:

```ruby
g.column_headers => ['Q1', 'Q2', 'Q3', 'Q4']
g.rows[0].data   => [obj_1, obj_2, obj_3, obj_4]
g.rows[0].column_data('Q2') => obj_2
g.rows[0].column_data(42) => nil
```