Replace the column_family attribute with table_name, for Rails compatibility

Add docs to the README showing that the option exists.

The old column_family getter/setter still exists as an alias, but a
deprecation warning is also emitted.

Fixes #6.
Remove unnecessary comment
Merge pull request #7 from data-axle/replace-column-family-with-table-name

Replace the column_family attribute with table_name, for Rails compatibility