Add method to get generator for iterating rows.

[monokrome]

This will return a generator providing each row in the workbook. This is useful, for instance, when using map to map a function to each row.

As a secondary note, shouldn't these comments be docstrings? It'd make this tool a lot easier to use from the shell.

[loamhoof]

Any chance to see this merged at some point? It's simple enough and remove the need to use an ugly range.
Though @monokrome, I would rename get_rows to rows to match the naming of the rest of the API.